Here are two ways to do just that:
First, the manual way - Reed Hoffman from Blue Pixel has an interesting post about creating easy photo collages using Photoshop or Photoshop Elements. This method gives you all sorts of control on how your images are placed, what size they are, and which elements are emphasized.
But there's also an automated way to make posters that include lots of images (if you have a Mac, that is) - a piece of software called Posterino that takes an iPhoto album, places the pictures into a poster layout and then gives you some control over placement, size and cropping of the images. It even outputs JPG and TIFF files, and interfaces with the Kodak print service from iPhoto.
Posterino is currently in Beta and costs $19.95 (a $5.00 discount off the final release price). A little steep for a Beta perhaps, but it is a very cool piece of software if this is what you're trying to do.
For tips on taking great pictures for your posters or collages, visit www.justshowmehowto.com!