GIMP bump map tutorial
Create a Gaussian blur
Apply a Gaussian Blur to the image, this softens the edges of the logo on the finished bump mapped image.
Filters » Blur » Gaussian Blur
The default Blur Radius values that will come up in the Gaussian Blur toolbox are 5.0px for the Horizontal and Vertical values. This was too blurred for a watermark, so we adjusted them down to 1px for our images, but feel free to experiment.
You can now save the complete logo template. We found that saving it in GIMP's native .xcf format worked best for preserving the logo's blur effect.
That's the hard work done, now that you have the template it's pretty straight forward to use the bump map tool to watermark your photos.