As the title says it, I've been trying to submit a wallpaper for several times, but it gets rejected every time. The submission rejection page just says 'Reason: Undefined reason.' It's hard to fix an upload when you don't know what to fix... I've changed the upload, file, image, name, description... Now I'm starting to wonder what else can be changed... wallpapers don't require anything, do they?

There just aren't any special instructions for single wall submissions, and a while ago, I managed to upload a set of wallpapers without rejections.
The wallpaper, I created in Paint, works well in my game and hasn't got anything strange. I'd just like to know what is the most common reason behind this mysterious 'undefined reason'.
However, I managed to mess up with the file on my sister's Mac, and it takes time before I get back home re-uploading.