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Clearly your 13 year old stepson should not have gone through your things and did something very wrong with your memory card. The fact he blurted it out shows he didn't realize how wrong it was.
But clearly, you should be old enough not to have pictures of your erotic self lying around the house. This stuff happens.
You are sharing the house not with "his" children, but children, period. I am sorry you were naive and that it is painful, but you do have control over the situation.