how they prove that you actually put the image? It can always be a hacker...
The one who has to pay up or get sued is the one responsible, not necessarily who put it up. However, if you can prove that someone else is responsible, as you said, a perfect example would be a hacker, and you had no reasonable knowledge of what happened, then you may still be sued and you would have to sue the third party and good luck with that!
In other words, you can't just say that it was someone else's fault and the copyright holders should go after them, if it's in your name, you're primarily responsible.
Just to give you an example of a case where it's unlikely that you would be held responsible, think about a well maintained website with a reasonable amount of security measures, including continuous monitoring, where a very skilled hacker breaks in and posts a copyright images. If you don't fix the problem within a reasonable amount of time, I don't know, maybe an hour, maybe a day, maybe 5 minutes depending on the circumstances, then they would almost certainly lose a case and wouldn't sue.
BTW, I'm not exactly a lawyer, take this info as my personal opinion and therefore with a pinch of salt!!!