Besides the program falcon listed for you, another freebie that can do layers is The Gimp http://www.gimp.org/windows/

If you don't need layers, then PhotoFiltre is a much easier to use, free photo editor program. http://www.snapfiles.com/get/photofiltre.html It even has a few useful plugins at the author's site.

FastStone Image Viewer is a handy image viewer with a bit less editing capability. But it adds some very nice touches like batch re-naming, re-sizing and so forth. The newest 2.4 version added text and line drawing that could be used for your thought bubbles. http://www.snapfiles.com/get/faststoneviewer.html