PS: I got the idea from this Instructable
It's a great instructable but here is a different (perhaps easier) way to do it.
Step 1: What You Need
GIMP (free) http://www.gimp.org/
a digital camera - optional
Step 2: Editing
Open it in the GIMP
Click file...open as layers...and open the same image
Select one of the layers and click colors...desaturate.
Drag the grayscale image to the bottom layer.
Step 3: Color to Alpha
A window will open, copy the HTML color notation then close the window. With the colored layer selected click colors...color to alpha. A window will open, click on the bottom bar. Another window will open paste the HTML code there. Click OK to both windows. Keep doing that until you have removed as much background color as possible.