At Wincustomize.com [and other skinning sites] the concept of 'using another's work' pertains to borrowing parts [or all] of an image [or skin] and adapting it/using it to create a skin to 'match'.
In the case of wall images they might be included with the matched skin upload [with consent] so, eg. You download a Windowblind by one skinner and it includes a Wall by someone else.
You might also seek permission for a wall image to become a Logon or Boot screen - both of which are not 'specifically' wallpapers themselves but have added modification/s for the new function.
The issue we tend to find inappropriate is when someone [here] finds a wall image somewhere on the net...and just uploads it via their own account - with or without consent. There IS NO 'value added' improvement to the product...no adaptation of an image to a different funvtion.
When a wall submission is simply someone else's image we prefer that person to submit it themselves - particularly if and when the submission is rejected - as then it may reflect poorly on the part of the uploader and perhaps even embarrassing for the original artist.
If people see artworks elsewhere and think they'd be ideal/accepted by Wincustomize then simply contact the artist with that suggestion and leave the decision entirely to them...