Color to alpha action

Automatically make an icon's backgroud transparent

I would like to have the ability to replace the background of an icon with transparency. Typically, an "icon" is an image surrounded by a fixed color. It would be sufficient to use the top-left pixel as a hint of the background color. Then the program could select all of that color surrounding the icon and replace with transparent pixels (or create an alpha mask of the negative shape, or some manipulation like that.) Output file should be a PNG; input could be a PNG, or optionally any ordinary graphic file format. Thanks!