One of my upload has been deleted also, with a resolution of 1920x1200. It was a low light photo with some noise, i just thought it was acceptable. I read carefully the upload rules, there is nothing regarding what is a "low-quality images". I found this in another thread:
Gandalf said:
Generally the resolution should be as high as possible. People can always scale them down for their own use, but scaling up will cause a loss of quality. If you set a wallpaper on your desktop it is usually scaled down automatically.
However the big exception is, that any wallpaper you upload should look good at full resolution, i.e. at 100% zoom! For photos this is very often not the case in their raw resolution, because most cameras cause some level of grain. In that case you should try to get rid of the grain and you may have to scale the image down far enough for the grain to disappear.
TL;DR Scale it down as little as possible but as far as necessary to get a good quality.
If you upload your own photos you should also read this thread about uploading your own creations.
I think having these guidelines in upload rules would be nice.