Therefor you don't need to insert different images.
It also can be done with a dingbat like the font "Windings".
Here an example:
https://acrobat.com/#d=ggwYgD9o-*JsWHfxtNCELA
It has a textfield and a Text with a simple circle as symbol.
A script in the layoutReady:event of the textfields checks if the textfield is eiter empty or filled and changes the font color of the symbol.
if ($ eq null) then
Signal.font.fill.color.value = "255,0,0"
else
Signal.font.fill.color.value = "153,204,0"
endif
![Signal_Icon.jpg Signal_Icon.jpg](/t5/image/serverpage/image-id/1690i454CE6378B4DFDDD/image-size/large?v=v2&px=999)
radzmar
http://thelivecycle.blogspot.com/