Watermark Software for Macintosh

I'm having difficulties finding a FREE and good watermark software for my pictures in my macintosh. I currently using Visual Watermark software but the quality produced by the software is bad. I don't like it at all. It deteriorates the original picture badly, you can see some photos with watermark on it which have bad quality rather than without watermark on it. Can someone recommend something that is FREE and good? =)