It's called posterization. Essentially you're reducing the number of colors in the image, and it's the color reduction settings in a photo editor program that will create that effect (combined with careful use of the brightness and contrast controls).

The trick is that if you take just any old photo and simply posterize it, it's not going to look nearly as good as the Juan Valdez or Flying Tigers images. Those started off as a simple posterization, but were heavily tweaked by hand illustration to turn them into something really appealing. I think both of those logos predate computer imaging, so the posterization and hand illustration were probably truly done by hand.