What application can generate an image mosaic of my other images?

I’m going to have a hard time explaining this, so take a look at this website: http://click7.org/image-mosaic-generator/?create

It takes an image provided by you, and then recreates that image with a lot of other images. Now, I want to do that, but with my own images (I have a lot). Does anybody know of a tool to do that? (If it’s a desktop application, I’d prefer if it works on Ubuntu.)