How to create these images:

1. Use Gimp
2. Create a Black&White image of your icon where the image is large enough to keep a duplicate of itself
3. Duplicate it
4. Use the gaussian smoother with a with of 3 pixels
5. Overlay the original on the new image (use "Nachbelichten") and merge the two images
6. Use the Bumpmap feature of Gimp (linear, invert) and move the bump-map to the empty space in your image
7. Cut out the bump map
8. That's it! 