You considered using the same illustration for your second book of the trilogy, only on a blue instead of yellow background? You've set a sort of precident with your link, and I bet the T-shirts would be big movers.
That also leaves the question of what to use for the third one. Blue + Yellow= green, but red might be a possibility too. [Just thinking ahead...]
It's not likely, but just on the off chance that EFAD takes off like the next *All your base are belong to us* you need to be ready to jump on top of it and surf on the wave.
-archy-/-