Digitized images can saved in a multitude of file formats.
Many formats compress the image in some way to make the file size smaller.
Several graphics formats provide reasonable compression and are cross-platform compatible. Not every format is compatible with every program. Graphics compression can be lossless or lossy. Lossless compression looks for repetitive patterns, but doesn't change the image. Lossy compression also looks for repetitive patterns and averages parts of the image to make it smaller.