Also, I just checked. The button images can be named whatever you want, you just have to reference them by the correct name in the INC file. So, for example, the first button by default is this:
[MeterBUTTON1]
Meter=BUTTON
X=0r
Y=10r
ButtonImage=images\portal.png
ButtonCommand=!Execute ["#B1#"]
You would change the second to last line that says "ButtonImage=images\portal.png" to whatever image file you want to be displayed. For example:
ButtonImage=images\chrome.png
Just make sure you place the image in the right folder.