greg2 ...any upload, skin, wall or 'just' a screenshot requires a 'full-size' jpg image as the preview. The site generates the smaller one for you [which means it can be clicked on to show the larger].
Even a screenshot upload means you just upload the same image [full size jpg] to both 'file' and 'preview'.
If you currently have failed uploads can you mention which 'section'?....are they walls, skins or what?...