I agree with Justin on this one... you could, in fact, write a quick function in php that could work universally throughout your entire site.
The best way would probably use PHP Get Image Width command, for example, because if the image does not exist, then the Get Image Width will return False. You could then set a condition where if the Get Image Width is false, then to pull a default image.
That should get you started. I would go into more details but I am short on time today.