How I made my mosaics...
I created a flickr account. (It's really easy for a person who uses blogger because they just transfer your information.)
I uploaded photos
I organized them into a set
Then I went to http://bighugelabs.com/flickr/mosaic.php
From there it is really easy. You can make your mosaic as big or small as you want. You can use a flickr photo set, or your flickr favorites or you can use individual URL's.
After it makes the mosaic you can right click on the photos. "Save photo as"... and then go back to your blog and "add image"
I stole this idea from Ali Edwards blog and I am soooo lovin' it!