We're currently in the process of getting our own tilesheet made - this was simply the first one we found several months ago. I'll add a disclaimer to the post, and hopefully by the next iteration we'll have our own original tilesheet.
"However, does the fact that the image is meant to be changed mean anything in terms of legal issues?"
Haven't much experience in copyright/laws to comment on that. Had a discussion on sprites/tiles once on SO, http://stackoverflow.com/questions/143050/where-to-get-sprit...
Reusing copyrighted images most likely doesn't count as "fair use".