At first blush, I think one could easily have the attitude that a picture of a key is no more dangerous than a picture of a gun. The vulnerability is clear but not completely obvious. Especially when prisoners can also see the real keys, as guards no doubt have to use them near prisoners from time to time.
Given the description of the key looking like an E, the picture could have been an image of an ancient old key and the actual key was an ancient old key. Old locks with that E style shape are very easy to open due to being so warn. We used to pick them at school using modified spoons and such like. I'd hope the prison wasn't like this though.
