Well watermarked images are allowed. If you want to credit a wallpaper maker or an artist go ahead. But remember that the artist is the one who draws the girls or boys or whatever is in the image. The wallpaper maker is the one who applies some brushes and things of the sort to make a wallpaper out of the original image.
For example we have post #16903 . That is a wallpaper made by someone. That someone did not draw the girl. He simply edited the image. The original image from which that wallpaper was made is here. The artist is marked on the left with a yellow tag.

In any case, it's kind of pointless to credit a wallpaper maker since they credit themselves on the image @ signed and watermarked. So just credit the artist of the original image if you want to. We don't ask anyone to credit anyone. It's all up to them.