Yes, it’s unfortunate, but people will steal your pictures eventually. I have watermarked my photos in the past, but I don’t now. For the watermarking to be effective, it has to cover the actual cake, and I just didn’t like the way it looked. Anything less can be cropped out too easily.
I will sometimes use TinEye.com or Google Image search (just drag your image over the search entry) to see if someone has stolen a particular image.