Obama Poster Script
If you are looking to do something like this above, I have found a script to do exactly that. It's pretty simple other than you need to find the right settings in the dialog box that will work for your image. There is an undo for the script but it's just easier to delete the layers if you don't like the results. Gimp can be fussy on "lesser" machines, so if your computer gets laggy from Gimp, back out of the script by deleting layers or simply closing out the image and starting again.
I've just discovered the script today (even though it's not new) and have only used it twice. I'm pleased with the results. Hats off to the writer of this script, whoever you are (the site doesn't give his/her name).
It's a great script. To "install", see the link above left on installing scm files. I am using Gimp 2.2.1, and I found the script in Filters> Artistic> Obama Hope. It is not under script-fu.
I mentioned settings. There are several to tweak but there's also the ability to change colors. This script is similar to the cutout script except that it recolors the image and also adds a stripe layer (not seen in the image above).
To get the script, go to the Gimp Registry Blog here. The download is under the word "attachment" in a grayed box. Find it in your downloads folder and copy/paste to your script folders (I put it in both found under User> Name> .gimp and Program Files> Gimp> etc etc etc.).
[Image Source: Google, a joke in reference to Clint Eastwood talking to an empty chair at the RNC in late August 2012.]