First of all I gotta say I love the design! We had a similar issue with our buttons that were supposed to change background image on rollover. In our case it was due to the second or third image that was supposed to show up not loading in time so the interaction looked glitchy.
Way we fixed it was by layering the button (background in your case) with all the possible images its supposed to switch to, then turning their opacity up/down as the rollover happens in each case. That way every possible image is loaded first, then faded in and out on rollover instead of having to be loaded on each interaction.
We achieved that via the “states” function of the designer, in your case I think you need more than 2 states so I suppose you can make multiple background blocks, each with its own image, lay them on top of each other and then fade them in and out with opacity, end result is the same as above.
Might help you out in this case as well.