A friend and I made an art prompt tumblr called Pinterest Design Challenge.
Basically, every week, we put a series of 5 photographs from our pinterest onto the tumblr, and then anyone who wants to participate can design a character that incorporates all 5 images! It's loads of fun and we've gotten some super cool stuff out of it! It's very laid-back and non committal, so no pressure to finish if you can't make it! There's no time limit. Just a new challenge every week!
Check it out!