nomacs

Nomacs is a very fast image viewer that can work with most popular image formats, including RAW, so it can open any image you want to see without any incompatibility problems.

It's completely free for Windows as well as Mac and Ubuntu and works under the GNU license.

Besides its main image-viewing function, the program also lets you edit images. You can, among other things, crop, copy, rotate, resize, and even edit in batches.

It also lets you compare two images, so you can find small differences between them, which is a pretty uncommon feature in other similar programs.