How to realize a battery-indicator with a trayicon? How to show the current capacity

[Rorschach <r0rschach lavabit com>]

Hi,
I wanna write a little battery-indicator in C with GTK. It's already working nicly in the console and I know 
how to write a trayicon with gtk but what I don't know is: how can I show the current capacity of the 
battery? I mean shall I do 100 pictures (1-100%) and if the capacity changes just show the corresponding new 
picture? This is imo a bad solution because I have to ship 100 pictures with my application. 

But how else can I realize this? Painting the image on-the-fly? Just showing the battery and painting it's 
content on-the-fly?

What's your idea with this?

greetings
regards
         Rorschach

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