[matthew]

I used to use a custom brush in Photoshop – the results could be good, but the process wasn't something I would recommend for more than a few images as it had to be applied individually. On the other hand, it does let you choose the appropriate size, colour, and position for each image, so it's quite effective if you can take the time and want the best results.

A photoshop tutorial that I found, but GIMP should be able to do something similar.
http://thephotoletariat.com/make-a-custo...watermark/
(essentially: create logomark, select, save as a custom brush shape from the appropriate menu.)

I've also used Lightroom, but now use Smugmug as a photo host, and it can automatically add custom watermarks to every image in a gallery. It's not cheap, but I have thousands of photos hosted with them by now.