Most of the time, when indicators are put inside buttons the problem is that the drawing of the button overrides the indicator drawing. I would suggest making your indicator either surrounding your button area OR outside the button OR make it BLINK (using :B).. this will ensure it's visible when ON...
Other than that, if you're really gonna load it from a file, the easiest thing is to load them with the IMG definition in the .skin file.. then just toggle it on/off as usual.. this would be a 2 button example:
/ Images (for the indicators)
in ExecTBL (in the skin's folder for instance):
Note that I defined the Indicators OUTSIDE the buttons, if you want them inside, then add :B in the quotes for the I1 and I2 definitions (for them to Blink):