Board Thread:General Discussion/@comment-1738746-20150719112753/@comment-303594-20150720123008

Tupka217 wrote: If they're the artist and upload it themselves, they've released it under CC-by-SA.

The best way to get answers on these matters is ask staff via Special:Contact/general. Not sure if this is true.

"Freely licensed or public domain images are strongly preferred on Wikia. Non-text media on Wikia should not be assumed to be available under the same license as the text. Please view the media description page for details about the license of any specific media file.  If you are uploading files to Wikia, you should cite the source of the file, attribute the authors, and note any copyright information, where applicable. Individual communities may elaborate upon and refine requirements for file uploads."

So it depends on the licencing given by the uploader when they uploaded the file. If there's no license I'd assume it's copyrighted, and so any use is under fair use.