Just for fun, I’m pleased to announce a working version of QtMosaic.
It allows to create a mosaics database and then generate a photomosaic from this database.

For instance, here is an original image:

The used database is freely available on the Internet, and the photomosaic is generated with 18×24 thumbnails:

The code is available on GitHub, as usual, and it has been tested on Windows and Linux.