There is an open-source (that is, free) image editor called The Gimp. It's quite a lot different from most other image editors so people might find it difficult to use. Make sure you also download the User Manual (and read it!). It is the only free (or even cheap) editor I know of that can save files with the correct opacity settings. The Windows version is at http://gimp-win.sourceforge.net/ and Mac OS X versions are at http://gimp-app.sourceforge.net/