I found a fun free program called Poster Forge that I downloaded and tested. It lets you create your own custom posters. You can print them out in full poster size because it will split the image up into separate pages that you can glue together to make one large poster. The image below shows how my poster will print into 4 pages for me.
You simply upload a photo, then add your own titles and text.
Poster Forge has three poster templates: Motivational, Movie and Wanted. Each template has it’s own customization fields. Just upload any image or photo on your computer.
Turn you and your friends into movie stars or put them on WANTED posters. If you like motivational posters, you'll be able to create your own!
Here is a link to their screen shots.
The only catch is small watermark on the bottom of the poster. If you want it without you have to pay for the full version. It’s a small logo and I think for a free program, it’s well worth the small logo.