I saw a tutorial for something like this in the November issue of Black+White Magazine and thought it would be a neat way to display my portfolio images. It was not too difficult to do, though I had some challenges with the file size when I finished - not sure about the best way to deal with that.
The steps essentially are -
1. Export all the images to one folder (makes them easier to find), resizing them to a square with a size of 15-20cm.
2. Open the first image in Photoshop.
3. Resize the canvas to be large enough to include all images, be sure to set the anchor to the top left corner. (I went a little larger with the canvas at this point and downsized it later once I knew how big I needed it to be.)
4. Open the next image. Using the move tool, drag it onto the canvas next to the first image. Repeat until you have all the images on the poster.
5. At that point, crop as needed and flatten the image. You can resize the canvas if you want to add a text box to the bottom as I've done here.
Lining up each image was the most time consuming part of the project. It turned out great though - I now have this poster of my portfolio images hanging on my wall. I like it enough that I want to find another 'project' to work on that lends itself to this kind of presentation. Too bad the edges of the poster get lost on this white background.