There are many ways to get your images printed into posters:
Zazzle.com - Have your picture or design printed as a poster, keychain, t-shirt, and more.
Block Posters - This free program lets you upload an image and save it for printing into a block poster. It's a pretty neat idea, especially for teen rooms and the den.
Poster Burner - Turn your photos into professional posters.
Also, if you are looking to print off multiple copies of your poster design, contact your local printing shops. So far, all my experiences have been good with my local shops, getting everything, from bookmarks to books to postcards printed.