At this point it's quite clear to me that they haven't even tried to make sure they are using a safe site. It also looks like the warnings issued were not taken seriously. In addition to the PM warnings about plagiarism, every time you submit something it clearly says on the submission page:
To submit a work you must be the creator of that work, do not submit other people's art.
So you are constantly reminded that such an action can get you in trouble. We have also clearly stated in the terms of service what is acceptable:
Wallpapers can consist of modified screen shots (or official artwork / manga scans) or your own artwork, but cannot include the artwork of others. This is plagiarism.
So basically, all you have to do is find sites that host scans from published artbooks for video games, or anime shows. Or even published artbooks for artists who draw in anime style that isn't in any series. Places like AnimePaper and MiniTokyo have sections that host thousands of scans that you can acquire. There are other sites on the web that also host artbook scans.
However it is important to try and look into the locations you are using. If there isn't enough information or it isn't clear, simply do not use it. Then you never have to worry about it.
Final thoughts will be on page three.
