File optimization can dramatically reduce the size of image-heavy files - by as much as 99% in some cases. It works by eliminating any excess baggage and converting graphics to the most appropriate file format and resolution. Optimized files remain in their original format and retain all their functionality, so they can be opened and edited by anybody. There's no need for special decompression or viewing software.
See What does NXPowerLite do to my files? for more information.