Sun May 17, 2015 7:38 am by lax
Bob's on the money here.
Unlike actual buttons which have a distinct "press" and "release", the mouse wheel just scrolls and at that point it's "hey I scrolled the wheel". ISBoxer turns this into a pseudo-button for Hotkey purposes, where you scroll and, at this instant, it performs both the Press and Release of some Mapped Key (or Menu button, etc). There is no amount of time between the Press and Release for you to see the image change to a pressed state.
The solution is pretty much as Bob suggests, with a timer to set the image later on. Note that MiRai ran into this same thing when setting up his virtual mouse/keyboard overlays; I believe his delay was more like 0.8 so that viewers have more of an opportunity to see it.