Apples are like an addiction to him.
I'm not sure if he still ate them in the shinigami world, but in this world, if he doens't have any, his body twists up, like it did before (thus Light having to feed him the apple) It MAY be that it's supposed to represent how Light believes himself as a God, and thus is passing the temptation onto others?
But there is also little to no allusions to religion and God in the series, save for mentions of it. there's nothing blunt, such as in Evamgelion, so it may just be a slight "pun" (for lack a better word) when he picture is there, with Light in Adam's place, and Ryuk taking up the other position.
One quick little tidbit of information for ya: There actually WAS no apple mentioned in the bible during the fall of Adam and Eve. It only says "fruit" when referring to the tree of knowledge of good and evil, and so it could have been any fruit, rather than an apple.