I use Gimp2 to edit icons and pictures. Heres a transparent icon if you want to give it a try. Rename them stat_sys_battery_100.png through stat_sys_battery_0.png for the discharging icons. Then stat_sys_battery_charge_anim100.png through stat_sys_battery_charge_anim0.png for the charging icons.
Heres the transparent icon. Its already sized the same size as the current battery icons (38x38) http://dl.dropbox.com/u/12608435/transparent.jpg
This is what to look for when using Gimp2.
If you try to make your own, let me know how it goes. I'll help if you need it.