Hey sadsack,
In a graphic editor that can save png files, you'll want to select all pixels that match the background color, and delete them.
Below are instructions for doing this in both Photoshop, and in Pixlr (a free online Photoshop-like editor).
The instructions are almost identical for both programs.
Photoshop
In Photoshop, you can do this with the "Magic Wand" tool:
- With the image open in Photoshop, double-click the image's layer in the layer panel to convert it from a background layer to a normal layer. (Background layers can't have transparency.)
- Choose the Magic Wand tool, and in the options bar (top of screen), disable "contiguous" and "Anti-alias", and set "Tolerance" to "0".
- On the image, click the background color to select all pixels matching that color.
- Press delete to remove them. This leaves a transparent hole where they were.
- Save the modified image as a png to preserve the transparency.
Pixlr
In Pixlr, you can do this with the "Wand" tool:
- Choose the Wand tool, and in the options bar (top of screen), disable "contiguous" and "Anti-alias", and set "Tolerance" to "0".
- On the image, click the background color to select all pixels matching that color.
- Press delete to remove them. This leaves a transparent hole where they were.
- Save the modified image as a png to preserve the transparency.
Result
Since I walked through the process in both programs to make sure I wasn't forgetting anything important, I ended up with the resulting image, which I figured I might as well include.
[attachment=0:23rp6ggf][/attachment:23rp6ggf]