This looks like a Lichtenstein painting brought to life by the magic tablet from Night at the Museum. That robot has a giant gun and a volleyball bomb, and he appears to be rocketing toward us on the wings of an anus-flaying thunder s**t.
The expression on that ghost's face is pretty much how all of us felt after staring at that cover for 20 minutes on the way home from Toys "R" Us only to be rewarded with a detonating Kubrickian hedge maze once we finally plugged the game in. Also, I'm not sure if the pumpkin and the raindrop are enemies, or if they're just kind of lost.
Don't get me wrong -- Bomberman is a fun game, but that doesn't explain why the cover looks like a terrorism postage stamp when the character you control in the actual game looks like this: