So, I did a search on the English Wikipedia for the text "welcome", and I restricted the results to multimedia.
When I did this, the first result that came up was [[:File:Anus.jpg]]. (Warning: neither the search link nor the file link are safe for work.)
The file description page doesn't have any mention of the text "welcome", and I couldn't spot any other reason why this image should turn up for a search for "welcome".
Also, the same search on Wikimedia Commons does not return Anus.jpg as a result.
My first thought was that it could be MediaWiki's version of a [[Google bomb]], but I don't know if this is true or not. Sorry if this is a false alarm, but this is a very strange result to get from this search, so I thought it was worth reporting here.
Version: unspecified
Severity: normal