I recently created the OCAU Avatar Mosaic. A mosaic of course is an image comprised of smaller images - in this case all of OCAU's forum avatars on a given day were used to create a large OCAU logo. It turned out quite well and if you haven't seen it before I encourage you to check it out.
You will also need an appropriate image, with many colours in it, such as the one I used for OCAU:
It is important to note that you don't need a high resolution image, just one with as many colours as possible, as that tends to lend itself better for adapting avatars into the mosaic. It took me quite a bit of mucking about till I got good results with the OCAU avatar, as each time you change the image, you will need to re-make the mosaic.
Step 1: acquire and format the images.
As I was making a mosaic from OCAU's Forum avatars, I used wget to download them all from OCAU's server. Once you have your images, you will need to convert them to JPG as that is all Andrea Mosaic uses. I used IrfanView to convert the images thus:
Open IrfanView, and go File -> Batch Conversion
Navigate to your avatar folder, and create a new folder named 'jpg'
Select all the avatars, and click Add
Set the output directory as the 'jpg' directory you just created, and choose JPG as output format
Click the start button, and a few minutes later it will be done!
You now have all the images in a useable format!