Just a little update:
I haven't yet caught anybody stealing these images, but that'll probably take a while. (I have loads of lessons, and have only just started marking them. I'm sure images are being stolen as we speak, but the odds are that it will be unmarked images that are walking out the door.)
A couple posters mentioned that the copyright notice could be edited or cut out. To avoid cutting, I've figured out to do the copyright notice like this:
This will make the "cutting" process considerably more difficult. However, the simplest solution (for those that catch the problem) would be to remove the image's transparency and convert the (currently pink) background to white. Then the copyright notice disappears.
However, I am dealing with educators, not rocket scientists, and these people usually don't know their way around a graphics program (that's why they're stealing my images). I know that this technique won't stop anybody from stealing, but it does tend to make it easier for me to justify my claim of ownership after the thieves have struck.
Note to "Rhys": "Eliz." is the proper abbreviation for "Elizabeth". I'm a "she"!