Well Mike, you've got me printing again! Thank you very much for spending so much time experimenting and explaining things to me. As you pointed out, I was confused about image size. My thinking did indeed use image file size rather than the memory size needed by the print driver. Learning the difference cleared up my thinking a lot.

As you suggested, I changed my workflow a wee bit after editing is completed by (a) reducing pixel dimensions from 25,000 x 20,000 x 16-bit  to  20,000 x 16,000 x 16-bit, (b) reducing pixel depth from 16-bit to 8-bit, and (c) saving the image in an empty folder on my desktop as a TIFF file. I set QImage-Ultimate to open that folder automatically to limit memory problems that may arise from larger image files present in my other folders.

Thumbs are displayed properly and images print properly so I am happily printing again. Thanks again for your help, Mike.

Thank you, Mike. I'll give it a try today and report my results late today or tomorrow.

Thank you for your suggestions, Mike. I did some experiments yesterday to get more specific information. I started with a typical (for me) black & white flattened TIFF image (834 MB) and then used PhotoShop to downsize and save it in TIFF and JPEG formats. Here I am reporting the performance of QI-U displaying thumbnail images and printing the images.

23,482 x 18,629 x 16-bit TIFF (834 MB):  Thumb IRE  Print NA   
23,482 x 18,629 x 16-bit JPEG (834 MB): Thumb ok    Print AV
23,482 x 18,629 x 8-bit TIFF (417 MB):    Thumb IRE  Print NA
23,482 x 18,629 x 8-bit JPEG (417 MB):   Thumb ok    Print AV
20,000 x 15,867 x 8-bit TIFF (302 MB):    Thumb IRE  Print NA
20,000 x 15,867 x 8-bit JPEG (302 MB):   Thumb ok    Print ok
16,000 x 12,694 x 8-bit TIFF (193 MB):    Thumb IRE  Print NA
16,000 x 12,694 x 8-bit JPEG (193 MB):   Thumb ok    Print ok
14,000 x 11,107 x 8-bit TIFF (148 MB):    Thumb ok    Print ok
14,000 x 11,107 x 8-bit JPEG (148 MB):   Thumb ok    Print ok

NA denotes "Not Applicable" That is, image could not print without thumb displayed
IRE denotes "Image Read Error" and no thumb displayed
AV denotes "Access Violation at 0040DF95" and print failed

THUMB DISPLAY
Thumbs were displayed for all JPEG images, even an image as large as 834 MB.
On the other hand, a thumb was displayed for only the smallest TIFF image (148 MB); Image Read Errors were obtained for TIFF images with sizes 193, 302, 417 and 834 MB. This is a serious limitation since images can not be printed if their thumbs can not be displayed. For me, it seems to mean that I can not print images with a TIFF format.

PRINTING
Printing was attempted for all JPEG images and JPEG's with sizes of 20,000 x 15,867 or smaller printed okay. This is consistent with your recommendation to keep images under the 20,000 x 16,000 pixel size limit. It is too bad that larger JPEG images failed to print, however.
The inability to display thumbs for most TIFF images prevented me from printing them. However, the 148 MB TIFF image which displayed a thumb properly did print properly.

The most puzzling thing for me, is that I easily printed all of my TIFF images routinely using my old version of QImage. I don't recall if I printed my images with 16-bit pixel depth (850-900 MB) or 8-bit depth (425-450 MB) but I definitely did not downsize the large pixel dimensions of my images. The memory problems that I am now experiencing have occurred only after I upgraded to QImage-Ultimate. It may be that memory use within Ultimate is different than with older versions of QImage. I wish that I still had my old version of QI to do a comparison but I experienced a system disk failure during my 'perfect storm' when my motherboard failed and I lost my programs at that time (my user files were backed up). So, I upgraded to QI-U at the same time that I rebuilt my PC, made an OS change, and made a host of other changes. That is unfortunate because it makes it considerably more difficult to guess about the cause of my problem with QI-U.

Mike, I think I've given you an awful lot to think about. Sorry, but I can't figure this out myself.

This is a difficult one!

First of all, thank you for dealing with this problem, Mike. I appreciate it.

I drum-scan black & white 4inch x 5inch film at 5,000ppi and 16-bit pixel depth in TIFF format. Consequently, the maximum resolution of my initial digital files are 20,000pix x 25,000pix x 16-bit or 1GB. Of course, cropping reduces that and the majority of my initial digital files are 850-900MB. Editing is done in PhotoShop (with no problems) using its large document format, PSB, and then flattened back to 850-900MB and saved in TIFF format for printing.

Print sizes range from 4inch x 5inch (greeting cards) to nearly 4feet x 5feet. Most prints are in the 20-30inch range on the short size and printed at 600ppi.

My computer box has 16GB of RAM and I have not experienced a single PhotoShop crash so no memory problems there. I have printed these same kind of images with QImage exclusively for about 9 years and QImage has worked fine so I wouldn't think that memory is a limitation. I have to say that I think QImage is terrific.

My problem has surfaced recently and there could be many possible sources for it. Unfortunately, I experienced a 'perfect storm' and was forced to make several substantial hardware & software changes during a short period of time so I can't eliminate possible problem areas from the changes one-by-one. The motherboard in my PC failed and I did a complete rebuild of the PC box. Its operating system was upgraded from Win 7 to Win 10. I had to upgrade PhotoShop to CC. I had to replace my monitor calibration device since the old one was not Win 10 compliant. My 9 year old printer (Z3100) finally gave it up and I replaced it with a Z3200. And just for good measure, I got divorced and lost many of my passwords and software support documents in the process!

Thanks again for your help, Mike.

Should also have mentioned that I have had huge problems displaying thumbnail images in QI-U. I am under the impression that this problem also is related to a memory issue. I received "Image Read Error" messages for most of my images (most are black & white flattened TIFF files of nearly 1 GB size). Get "Image Read Error" with TIFF, PSB, PNG and large JPEG formats. I tried many things but to no avail.

Reduced the number of multi-threading cores used to create thumbnails.
Made sure TIFF images were flattened without layers, alpha channels, etc and only with a Gray Gamma 2.2 ICC profile.
Tried converting to various image formats (e.g. PSB save as TIFF save as PNG save as TIFF again) with and without flattening before each save.
I tried several other things but can not remember them since I have been dealing with this problem for quite some time.

Since I needed to get some printing done, I finally was able to display thumbnails when my big, beautiful 1 GB images were dumbed-down to about 100 MB JPEG files. However, then I encountered the Access Violation message that began this inquiry ... 

Grrrrr 

I am using the latest version of QI-U, v. 2017.118. However, the problem began before I updated to the latest version.

Image does not display in Comparator. Instead, shows message, "Not enough storage is available to process this command." Not sure if this refers to RAM or hard disk space, but I have plenty of both in my PC.

I have attempted to print both TIFF (nearly 1 GB) and JPEG (as small as 80 MB) images and obtain the Access Violation message with both.

I normally use print profiles made internally by a Z3200 printer. So, I deselected the profile and selected "Let printer/driver manage color" in the QI dialog and the print driver and still got the Access Violation error.

Need to correct a previous statement. In my previous post, I said, "Get an access violation message shortly after any image begins loading to Print." I should have said, "Once the print command is given, QI displays 'loading image' and then 'processing image' and the access violation message appears a couple of seconds after "processing image is displayed." 

Get an access violation message shortly after any image begins loading to Print: "Access Violation at address 0040DF95 in module QImage.exe. Read of address 00000000." 
Help>Analyze:  shows no conflicts
Help>Analyze(shift):  shows Start 2.146 GB, Add 0.909 GB, and Now 2.146 GB. PC has 16 GB RAM. Error message continues to appear when attempting to print any image, even images as small as 80 MB file (I usually print 1 GB files).
Help>ResetPrinter Settings:  Same access violation message appears
Help>ResetAllProgramSettings:   Same access violation message appears 
Uninstall QImageUltimate and reinstall it:   Same access violation message appears

I am at a dead end and getting frustrated. Any ideas would be appreciated.