blends two images where the watermark image can contain transparency information. If all your blending information is in the alpha channel (transparency) then you would leave the blending amount at 100%. To run the script from Safari just click on the link immediately above the script and it will open the script in Script Editor.
The script is probably more complicated than you would need because it can blend as well. Plus the script scales the watermark image to fill as much of the main image as possible without being cropped.
Also I have recently written a Apply Watermark Automator action which does the same kind of thing. It is included in the iMagine Photo Automator Actions downloadable from the automator section of macscripter.