For a birthday present I generated an image mosaic using the metapixel command.
Firstly get atleast 1000 images in [base_image_folder] and sub-directories. Then process the data with:

metapixel-prepare --width=64 --height=96 -r [base_image_folder] [tmp_thumbnails_dir]  

This processing will take a while.Then generate the mosaic with:

metapixel -w 48 -h 64 -s 8 --metapixel [image_pattern_for_mosaic] [output_image_file] --library [tmp_thumbnails_dir]  

This should only take a few minutes. For different resolutions change the size flag.