I find that there *is* a definite cut-off point, where images do supplant players' imaginative visualization of the scene. Depending on the style of game I'm running, that might be acceptable or even desirable--but it's definitely an important thing to be aware of while prepping.
Personally, the best "bang for the buck" I've gotten with visual/audio aids is when they are used as punchlines to create an "aha!" moment. For example, awhile back I was running a solo game over chat. The player had rolled a barbarian and was cake-walking through one of Paizo's dungeons (I forget which one). So he comes to this smokey room and blunders around for a bit, and then I tell him that cold hands are reaching through the haze to attack him. He assumes he's got more zombies to hack to bits and he's pretty gung-ho about the whole thing. So I say nothing, but just send him a picture of the shadow he's facing. Dead silence. Then he's like "Um.... .... That's not a zombie. Is it?"
The other great use I've found for visual aids especially is when one character sees something that the others do not. Again, sort of a punchline situation, where the effects are a dramatic, shared secret between the player and the DM. The player then has to decide what to make of what he sees, and how (or whether) to relay that information to the other players.