How to form a mosaic picture by using GIMP or another app. by using other pictures?
Solution 1:
Take a look at AndreaMosaic
Here is a guide that Engadget did about five years ago on this software.
AndreaMosaic, a free project to create digital art using images and computer software. With AndreaMosaic you can create your own photographic mosaics made with your own pictures. A photo mosaic is an image composed of many tiled photos.