Hi Gary,
9000 x 6000 is relatively small: only about 160 MB. Double that if it's 16 bits/channel which is still pretty small. Qimage can handle images 6-12 times that size easily.
So I wonder, what format are these images: TIFF, PSD, etc?
Also, do they have layers? Sometimes layers can cause issues and flattening images will prevent problems. I don't think it's the naming convention. It's probably just the order in which they are accessed along with some incompatibility with layers.
Regards,
Mike