No one has mentioned this yet, but talk to your MOM first.

As a guy, I know we have no brain. Even if your hubby is super duper loving and caring, he is going to be thrown for quite a loop when he figures out you know about his dirty little secret. He may try to lie his way out of it, or fib, or do something that ruins the marriage/connection.

Go to your mom and ask her about it, because she will be the most honest with you. It's your MOTHER, and if she has an ounce of respect for you, she'll spill. Then you can take her information and analyze that before talking to your husband.

We're guys - a lamp can make our dick hard. Yeah, it's gross that it just happened to be your mom, but - hey - we've all heard worse. The important thing to remember, OP, is that it was just a "fling", and that your husband is still with YOU now and not with your mother.

Just realize the reality of the situation. You are in love with this man, but you are confused now because you see pictures of his previous intimacy. But unconfuse yourself, because he doesn't feel that way towards your mother anymore. Yeah, maybe he has some fetish for mom porn, but...To be honest, that's his business, even if it is your mom. There's nothing illegal about holding on to porn.

But tl;dr - Talk to mom about it, relax, talk to some more people about it, let the situation blow over, then talk to your husband about it if you NEED to. Don't let this be a thing you blurt out in spite when your husband forgets to empty the dishwasher.

You - "Honey, did you get the dishes out?" Husband - "Oh oops I forgot dear I'm sorry." You - "JUST LIKE YOU FORGOT THAT THUMB DRIVE OF MOM PORN?" ^ This will cause a divorce.

If you value your relationship with him, slow and easy will get you back to normal.