*.gif

.gif is a fileformat mostly known for the fact that it can save animations. It’s a fairly old format, and it does it’s compression by indexing the colors to a maximum of 256 colors per frame. Because we can technically design an image for 256 colors and are always able save over an edited gif without any kind of extra degradation, this is a lossless compression technique.

This means that it can handle most grayscale images just fine and without losing any visible quality. But for color images that don’t animate it might be better to use *.jpg or *.png.