I was trying to make some figures, and google told me GIMP is a very popular software as an alternative to Photoshop.
Ubuntu 12.04 has GIMP 2.6 installed. To update to GIMP 2.8, follow the instructions here:
http://www.maketecheasier.com/upgrade-to-gimp-2-8-in-ubuntu/2012/05/04
and install the plugins:
http://www.webupd8.org/2013/02/gimp-284-released-install-it-in-ubuntu.html
It takes long time to make a good-looking figure.
I will write a post to demonstrate how to make a heatmap by R next time.