A number of flags here are 5000 by 3000 pixels large. I think that is way too large. They don't even have thumbnails and are replaced with the text "Error creating thumbnail: File with dimensions greater than 12.5 MP". I replaced the Foliage Bigender image with a scaled down version (1200 by 720) and it seems to have worked just fine. I will try to do this with the other non-working images (probably at 1250 by 750, though, 1200x720 is not 'exactly' 5 by 3)
Edit: I finished changing all of the previously non-working images to 1250x750.