Who painted this?

This quiz is for those who think they know their artists. I've designed this one specifically without Australian artists so as not to disadvantage those who visit from afar. I guess looking at my blog I am influenced by the abstract expressionists so I'll throw a few of those in for you. You may also find some of the answers in posts on my blogs.
This is only my second quiz -I'm learning as I go so please let me know what you think whether too hard, easy , ridiculous or boring.
Thanks. Good luck, break a brush!