Watermarker 1.3 (Mac App Store link) gives you a way to solve this problem by superimposing a mark of your choosing to an existing picture, thus “stamping” with your particular imprint.

I’m a bit behind in mentioning it, but Watermarker 2 is out. This Mac app from developer (and former MacStories writer) Don Southard lets you quickly resize and add professional watermarks to batches ...

Bronson Watermarker fills a small--but vital--niche for those who need to create watermarked PDFs and images. You can even export your files as a single image to make sure that someone can't strip ...