The image/photo mosaic plug-in creates a image/photo mosaic (composition
of multiple images) from an input image and an image archive.
It also has the function to create the image archive.
With GIMP menues, the plug-in is located at "Filters/Render/Image Mosaic".
It's help-button should tell you how to use the plug-in.

WWW: http://www.kirchgessner.net/photo-mosaic.html