What's a good Mac program/app that can easily watermark pictures for me?

I have some pictures of clroom worksheets that I'd like to put in my online portfolio, but I'd like to put a watermark over them so as to deter people from stealing them outright without my permission or anything. What is a good program I can use for this? I have Gimp, but I find it pretty thoroughly confusing to use...especially to try and do something this simple.