The default one is, currently, from Pokémon Gold or Silver. You're right - to use it under fair use we should include a copyright disclaimer. (However, does the fact that the image is meant to be changed mean anything in terms of legal issues?)

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.