Make a giant photo poster for free
I just made a huge printable version of this (this image is just a test and is only random photos)
Here is the full-size (3Mb) version of this image.
To create the above image, I opened Picasa, selected a folder of photos, then clicked the "Collage" button at the bottom. I chose the "Picture Grid" option and it cranked out the above image in no time.
To make one giant printable poster, I used the "Rasterbator" (I can't vouch for the work-safeness of this site). You can upload an image or download a standalone version and creat the printable posted on your own machine (if you are on a PC)
Once you've done this, you can print it off of your own printer at home.