GIMP?*

*If you haven't already tried it.

*has already tried it.

How exactly do you do accomplish that? I've tried, but I can't figure GIMP out.

You delete the layer you're working on (if it's got something to need, copy it all first), then make a new layer (set it as "transparent"). Then paste what you need onto that layer. Select the tool that looks like a little wand, and click on the area you want gone. It'll select all of that color and you can remove it.

Or you add an alpha channel and just erase the bits you want transparent.

Go to these.

Layer | Transparency | Add Alpha Channel

Also, if you want simple and don't mind it being more limited, not to mention having to do some forum-digging for plugins to get some features, try "Paint.Net".