VIPS's a free image processing system. It aims to be about half way between
Excel and Photoshop. It's obviously much smaller than these two, but it's
not a toy either (about 200k lines of code). It's very bad at photo
retouching, but it's great (much better than programs like Photoshop, I
think) at all the other things you use photoshop for. It's particularly
good with large images (ie. images larger than the amount of RAM in your
machine), and for working with colour.

WWW: http://jcupitt.github.io/libvips/