ranisalt Referring to your initial suggestion:
It would be useful to warn users about that and maybe provide a tool to help cutting wallpapers to reasonable resolutions, instead of just allowing anything.
Anytime something gets cropped we lose a part of an image that someone might have use for.
Let's say someone has a 1900x1080 upload, i.e. just shy of the common FullHD resolution. That resolution is somewhere in between the common 16:9 and 16:10 ratios. If we force users to crop it down to a 16:9 ratio that would mean cutting off a little bit at the top and/or bottom, so we'd end up with 1900:1069. That's even weirder, so next we could force them to scale it down to the common 1600:900 resolution. At that point we've lost a lot of pixels and someone who would have quite happily upscaled the original to FullHD (only 1%) will instead be disappointed by a significantly blurrier image (upscaled by 20%). Along comes someone with a 16:10 monitor. In order to avoid stretching the wallpaper he has to crop again, this time from the sides down to 1440:900. With access to the original he'd have to crop a lot less and only in one direction, down to 1720:1080.
So no, forcing uploaders to crop their wallpapers is a bad idea. Ultimately we'd just end up with people uploading multiple resolutions of the same wallpaper (one per common ratio, i.e. 16:9, 16:10, 5:4, 4:3, …). And then someone else comes along and wants the wallpaper cropped slightly differently and uploads yet another version… That's the situation we used to have and it was quite horrible.
Instead we offer a simple tool to crop wallpapers when downloading, as you already know. And, as you've correctly pointed out, this tool is fairly minimal and always crops to the center. This covers the vast majority of use cases, while functionality to cover the remaining few cases would mean substantially more work for us. It just isn't worth it. If you want more control over the precise cropping of a wallpaper there are plenty of excellent, free image editing tools out there.
I'd also like to point out that on most operating systems you don't need to manually crop/scale a wallpaper at all because the desktop settings utility will do it automatically.