I discovered however its not necessary to add a layer and what not.
All you have to do in Gimp is:
Copy the image (ctrl-c), and then paste it as new: (shift-ctrl-v)
Now erase the background and not needed stuff with the eraser tool (if needed resize the image and auto-crop it) and then drop the shadow you would like as desribed in Nicu”s Howto.
Then save the new image as a .png file and you are done.
To emphasize its simplicity I used Bush in this example.