Use a sharp exacto to cut out the image so that the gumpaste will form a backing support for it.
Make sure to remove all of the gumpaste from the small areas.
Attach the gumpaste-backed image to the cake. You might want to wait until it hardens up a little, but I just do it before it dries so that I can make the shape conform to the shape of the cake. Be careful not to rub the front of the image because the wafer paper might rip if it's still wet.
That will attach the image to the cake and give it some support. Even if it's taller than the tier it won't curl or slump.
Kara Buntin owns A Cake To Remember LLC, online cake supplies at www.acaketoremember.com and www.acaketoremember.etsy.com