I've been making mosaics for a while, using a host of different pieces of software on several platforms. Pretty much every mosaic has a theme for the thumbnails used to build the bigger picture.
To name a few: Todd and Felicia's wedding picture was built up from photos made during their wedding day. Balkenende's face was constructed from images of Holland. Mugabe's mosaic was created from scans of hyperinflated Zimbabwean paper money.