Could my History teacher get in trouble for this?

It really depends on how those pictures "were found". If he posted them on the internet himself, especially if he did it recently, it would probably be considered inappropriate behavior and he would, at the very least, probably be disciplined for it (whether or not he would be fired would depend on how the headmaster feels about such things). If someone stole the pictures, or if they were taken long before he started teaching, he probably wouldn't be in any trouble.