In Gimp you would just open it and click file, start new. Do a 80x80 for dock icons and 160x160 (what I use anyways). Then that box will appear. Right click the box, click layer, click transparancy, add alpha channel. Then righ click the box again. Click layer, transparancy, color to alpha. ................
Where the cursor is you have to open that and scroll thru until you see .png. It will ask if you want to export it and yes you do. There, you have now created an icon you can use on your phone.