It all depends on your jurisdiction. In the US, you have to have what is called "mens rea" in order to be found guilty of a crime. This means that there must be intent, neglicence, or recklessness in order for you to be guilty of a crime. You cannot be guilty of a crime you comitted by accident (unless there was recklessness or neglect). Thus, it's unlikely you would be prosecuted over a thumbnail you came across and didn't even click on. But with the law...you never know.