I would highly suggest you(and everyone else) get into the habit of using layer masks. They do take a bit more work up front, but give you FAR more flexibility in the long run(unless you working on a throw away thing that you KNOW you will never edit). Also, have you considered using GIMP 2.8(and if you are using it) and it's layer group feature?
I'm a total graphics/GIMP newbie, so thanks for the tips!
You may want to consider a Creative Commons license instead of Public Domain. You may not perfectly understand the differences, but if you visit CreativeCommons.org you can find some descriptive text comparing the various licenses.
Despite the thought of someone copyrighting a derivative, I still feel that public domain is what I want. Don't get me wrong, I use GPL/MIT licenses on all my code and love the CC licenses, but when I think back to why MapTool is so awesome, I can't help but think of the
mapping torrent everyone uses. Being able to quickly grab a single pack chock full of almost anything you need basically made this tool what it is for me. Putting something like that together is a lot of work though, even if you just have to abide by CC-BY. So the point is, I want to make it dead simple for someone to grab my work and throw it in something like that torrent. I don't care about being mentioned - as long as someone finds it useful. I'm willing to tolerate the potential for misuse for it to be easier to use.
Note: I am aware of the Dundjinni forums and RPG MapShare, but the point is ease of bootstrapping. If i'm a new user or on a new computer (I've had my collection of images lost twice now), I don't want to comb through forums or download the dozens of albums on RPGMS to get started.