Photo Jumble library file contains essential information about image files that will be used for mosaic collages. This info includes file name and location, image sizes, colours, colour distribution and so on. Also it contains a small thumb preview of every tile image. These thumbs are used for fast screen previews.
When the program exports collage into image file it use to draw re-scaled versions of your original images. That's why it is important to keep your original image files in the same place as they were when you build the library.
Photo Jumble may work with image libraries generated by Photo Jumble itself (they have .pjlib file extensions) and with libraries build with Image Mosaic Program Mazaika (these files have .mzlib  file extensions). Please see how to make library here.
The difference between .pjlib  and .mzlib:
Photo Jumble Libraries keep information about transparency of tiles and can be used for creating non-rectangular mosaic and collages. Mazaika Libraries don't respect transparency.