This is the mirror feature in the Image Editor - it does not have to be used image by image - as follows:
Select the thumbnails that require to be mirrored, right click and from that menu in the Edit Images section, select Predefined Filter.
Then choose {Q} mirror.flt. That will mirror all the selected images.
See screen shot below.
Terry

Yes I'm aware of those options, even selecting groups of thumbnails becomes tedious and contra productive, another thing I don't like is that you have to see them mirrored and a lot of this images are designs with text on them, and is just weird looking at them like that, also it makes them hard to spot when you are visually looking for an image by browsing them, I deal with hundreds (even thousands) of images, I have dedicated folders for every product, T-shirts, mugs, etc., if I wanted to see the thumbnails mirror I would mirror them at the time of saving with my image editing program very easily, what I would like is this mirror function to be part of the "Jobs Properties" so you have the option to apply the mirror function to a specific Printing setup, so when the image is printed the result is mirrored without ever actually see it mirrored... all I want is Qimage magic.

See the image for a representation for what I have in mind.

The other way is to use the Print Filter, in a similar way to the Global filter, the tick box is just below that.
With the Global filter, the mirrored image shows on the page preview/editor (which still may be confusing when there is text) but the Print filter effect only shows on the print.
You can of course save a global filter or print filter as part of a saved Print Set-up.
Terry