Welcome! The best thing to do would be to save it as a .jpg (I'm not sure if Paint will do this, you may need to download the Gimp - it's free - and open your original bmp and save it as a jpg). Then post a new thread in the Finished Maps section and attach it using the Manage Attachments button. If it's still too big you may want to consider creating a DeviantArt account. Many of our members post stuff there that is too big to post in the actual forums.
Can't wait to see it!