I'm always looking for painting challenges and through Horst Hittenberger, Karin Jurick and several other artists, I've come across some fun and/or interesting and/or challenging sites. Take a look and do what you will? Let me know if you decide to try any of them.
Lela Stankovic's http://paintanddrawtogether.blogspot.com/
Barb Benick's http://studioatelier.blogspot.com/
recommended by Karin Jurick http://www.rookiepainter.blogspot.com/
and one of the more interesting ones
where you paint from the google maps of the world
and one that looked wonderful but doesn't exist any more...