Originally Posted by AdultKing

If you are in the closed beta of the API you can do larger images. You'll need programming knowledge and experience using API's, you'll also need either a front end or command line tool to execute API requests.

There's also another service which is only API based that will do images up to 3 Megapixels that is also in beta (I've been testing it for about 4 months) which is being launched in February, cost per request will be about 7 cents.