Iris is a free, open source program for Windows only, that allows you to manipulate FITS files and save them to another format so that you can open your image in The Gimp, Photoshop, or another image editing program.

Iris can be downloaded from:

http://www.astrosurf.com/buil/us/iris/iris.htm

This pdf document will guide you through using Iris.

Manipulating FITS Files with Iris.pdf