mschleisner ...the wallpaper gallery is for wallpapers, nothing else.
Yes, photographs can work as wallpapers, but as with any graphic medium not all work as well as others.
One definite reason for rejections will be when the image is NOT in desktop/screen resolution, eg, undersquare...that means taller than it is wide. No computer screen is in that proportion, so neither should any wallpaper upload be.
This applies to anyone/everyone just the same, so it is not 'rudeness', just reality/restrictions of the gallery.
When it comes to subject matter, it is a reality that truly personal photographs will only appeal to the uploader. Sites such as Deviantart.com have plenty of photographs of 'my pet fido' which generally hold no public appeal [as wallpapers] at all.
At this stage, on Wincustomize.com we have an option to relocate 'less successful' uploads to a person's 'Personal Page', however in the case of many people's photograph uploads [in particular] the ideal option would be to be able to relocate the uploads to one's 'Photo Gallery' which even by its very name is a more appropriate destination......where odd-proportioned photographs/images would not matter as they were not trying to be wallpapers.
There IS no 'attitude' in my comments....only an attempt to answer your query ...to help a 'newby at this' ....