There will be an online auction organized by 140hours on Jan 29th where you can donate your work or you can bid and buy some artwork.
This online auctions site uses Twitter to bid, and if you want your painting to be auctioned you need to open Twitter account. On regular auctions they keep 10% and artist gets 90% of the final price, but for this auction I could not find out how much will they keep and how much will they send to Haiti. They just have general rules and if somebody has time to dig this out, please add a comment to this post.
You can also see the gallery of already submitted works for this and previous auctions.