Step 6: Assembling the poster in Picasa2
Download and install Google's Picasa2 (Installer, 5MB EXE)
Import your scaled images folder into Picasa2.
- Picasa Help
With the folder selected in the left pane, click the menu Create->Picture Collage, choose Picture Grid, click Create, wait a bit and you're done.
You end up with a square image that you can open up in Photoshop and crop/resize if need be.