hum, ok... i never evaluated how the blockers work... so it needs another wrapper around the ads block... with a different name etc... i suppose.. i would have to check it.

Ok then if the logic of how the blocker works being it blocks the ad code itself, then the alternate image code being inside the ad code, would be blocked also. Whew! Was that long winded or what?
Maybe put the alt image in the body background and use the z-index and position commands to park it where you want it.