I've just uploaded the latest in my series of articles about using RMagick. This one explains a very simple technique for creating a digital watermark that looks like embossed text. I hope you find it useful.
http://rmagick.rubyforge.org/watermark/watermark.html
The RMagick home page has links to the entire series: http://rmagick.rubyforge.org.
RMagick is an interface to the ImageMagick (http://www.imagemagick.org) and GraphicsMagick (http://www.graphicsmagick.org) image processing libraries that supports more than 90 image formats, including GIF, JPEG, and PNG. RMagick includes RVG, a 2D drawing API. Comprehensive HTML documentation for RMagick is available here: http://www.simplesystems.org/RMagick/doc/index.html and here: http://studio.imagemagick.org/RMagick/doc/.