Hi Ian,
GIMP (Gnu Image Manipulation Program) is the totally free image processing option:
www.gimp.org
It is not easy to learn but there is a free book you can download; much information online; and it has a vast array of features. The colour depth is only 8-bits per channel though.
I'm getting into it more and more every time I use it.
It's not easy to use. It is totally free.
Take care,
Doug