See that old man on the right, thats me rendered from a photo with the Gimp. Here is how I did it.
First open up a picture in the Gimp, most pictures are in .jpg format. Adjust the percentage to 400 or whatever looks BIG.
You want to create and edit paths with the path tool. What you do is just click along the edge to cut out your head. Just like connecting the dots. If one of the dots is off where you want it you can drag it to where you want it.
You can use the keyboard shortcuts but I just use the menu on the top of the image.
Go to layers -> Transparency -> Add Alpha Channel
then Select -> From Path
then Select -> Invert
then Edit -> Clear
then I save it as a .png <- make sure there is a check in the box for save background image
then Image -> Autocrop
Then I will Image -> Scale Image to around 100 pixels
then I close the image and reopen it. Not sure why I had to do this before I added the drop shadow but I did.
then Script-Fu -> Shadow -> Drop Shadow