Friend of friend here (also, I'm on the comm that has been watching this woman...)
I've tried in the past and there is no easy way to do it, especially at the volume I post things online.
Actually, the screen capture/editing tool I use (TechSmith's SnagIt) has a convert images feature that I use all the time. It's totally worth the $50 cost of the software. It lets you bulk process images as a batch, and perform multiple actions at the same time (rename, resize, watermark). I tend to take hundreds of pictures with my digital camera, and then have to resize them before I post them on PhotoBucket. It only takes a few minutes with SnagIt. And you can caption or watermark at the same time.
Just in case you want to start watermarking going forward.