If you have photoshop, you can also remove the background color altogether.
-Open the png in Pshop
-At the toolbar, go to Select > Color Range
-Use the eyedropper tool on the background color. Then set Fuzziness very low, like 10%. Confirm
Now you have the background color selected (watch out for selecting similar colors in the icon). You can remove it in two ways.
-One way: add a layer mask by clicking "add layer mask" at the bottom of the layers panel. Select the mask, and invert it by pressing Cmd-i. The background should now be blank. Save as a png with transparency.
-Other way: duplicate the locked original layer. Select the layer copy, and press delete to remove everything in the selection. Press Cmd-d to clear the selection. After that, select the locked original layer and delete it. Save as png with transparency.