What image sizes qualify to be added into the 'Wallpapers' section?

I've been finding the same problem for years. The information on the right of the Add A New Image page says that images 600x800 or larger should be ok for wallpapers (only those which are smaller are relocated), but I've had many which my computer says are larger go to Photos. Perhaps it's only a specific size, not anything above a minimum.

There's a way to stop having to edit an image to put it in Fanart though: click "upload fanart" when you upload it and then change the drop-down box to Wallpaper. It ought to go to Wallpaper if it's large enough but Fanart otherwise.