1. Import the pdf in Gimp
2. Make sure the resolution is at least 300dpi, best 600 pixels per inch
3. Import the signature as layer. It seems that it does it automatically if you copy/paste the picture from a world file.
4. You can move the layer when you paste it to fit it into position
5. You can rescale the layer to increase your signature (Layer -> Scale Layer)
6. When you're done, you have to merge down the layer to the page it belong to by first making the other layers (pages) invisible
7. Finally, export to pdf with the option to export layers and export pages in reverse.
That's it. The file is a bit big, but it will have a good enough resolution. If it doesn't, that's in image -> print size.
