Another way to generate a poster is with Picasa. Picasa is a free tool from Google to manage and edit Photographs. It links with their photo album website service. There are lots of things you can do with Picasa and one I was playing with today is generating posters. The posters generated are basically enlargements of the original photograph so the quality will be higher than a rasterbated poster but will also use more ink.
The screen capture videos for this were done with the new version of quicktime that comes with Snow Leopard. Pretty easy to use.
Update - Another online poster maker suggestion from Ban Ryan - http://www.blockposters.com/