Do you know about the application named Graffiti on Facebook? If you don't, then you should. It's really a very simple application where you have limited space, a circular brush, and a colour palette, and all you can do is change the brush size, colour, and opacity. You could also replay the process of making, for any picture that is made, and it's quite interesting to see how people use the brush. Recently, there was a competition named Posters/Wall Art, and the winners are here. My personal favorite is the one with the windmill, which I had also voted for.