I will show you how I implemented a toggle button with an image for an editor button. I know it isn’t directly answering your question, but hopefully it helps you find a way to do it.
I expected to have this: Redirecting… and I got something like the tool buttons of UE4 geometry mode (classic radio buttons, which are not good for tools). But I will try to do something like in the folliage mode (I’ll look at the source code).
Well I’m not sure how to do it from a coding perspective. But I’ve solved my button-image swapping issue using blueprints (maybe you can as well lol). It took some trial and error but the condition for the swap can be called based on an event dispatcher (which you may or may not need for your particular project.)