One of the joys of iPhoto is the ability to extend its reach with Apple Automator. Automator is a program that adds macro-like functionality to your Mac.
Automatorworld, a website full of cool Automator scripts features one by Jim Heid that I really like. It’s called Thumbnail Poster 1.2. It creates a multi-page 20″ by 30″ PDF document full of images from your iPhoto library.
What I really like about this is that there’s no need to manually crop anything, no need for Photoshop or custom paper sizes, and indeed, no need to do anything except click a button. When it’s done, you can order the poster through iPhoto’s printing service or use your own printer.
This does create a very large file so don’t expect to email this puppy to Grandma.
NOTE: This workflow will crop your images to a square format or let you select a horizontal or vertical format. You might want to make a separate album of images for this project that are either all square, all horizontal or all vertical. It will speed things up considerably if you select all horizontal or all vertical images.
Play with it and have fun.