I've run into the same problem.
I've been trying to make an image material into an emitter, but it won't emit.
Alternate idea was to make the image 90% opaque and put a white colored emitter behind it.
In both cases, whenever I go back to the materials editor for either material, "emitter" is unchecked.
I opened the ruby console and there are no messages.
I am using the demo in Sketchup 2016