in this tutorial, you'll learn how to turn this:

into this:


start by copying the original image into GIMP. user your layers dialog (this can be found under dialogs -- layers) to duplicate the original image.
with the top layer selected, go to colors - posterize and match your settings to mine:

your image should look something like this:

now, go to filters - blur - gaussian blur and match your settings to mine (again).
after that, set your top layer to Darken Only at 100%. add some borders, and some text, and you're done!