Until GIMP come along making a picture transparency is a very time-consuming effort! One of the old paint program that let you erase to transparency but it is hard to find now a day!
Here GIMP comes to the rescue!
. First, on the menu bar, pick "Layer", "Transparency", then "Add Alpha Channel" (Note: The most important step, otherwise it won't work!)
. Next on the menu bar, pick "Select", "By Color", then click on the color you want to clear to transparency.
. Last on the menu bar, pick "Edit" then "Clear"
Now when you save the file, it will be a transparency picture!
Here is a few GIMP related link: