Oh hun, what a tough thing to have to deal with

How much does she know about sex in general? If she doesn't really know much, seeing that kind of stuff can be confronting for a young child. Especially with the reaction from her dad and then you yelling at him later - she most likely thinks she's done something wrong and you're both angry with her.

Can you sit down with her quietly and explain that sex is something for grown ups and while it might look yucky/bad/naughty etc it's really okay between two people who want to do it and love each other but isn't for kids her age? I wouldn't say much more than that or make a big deal out of it, just be straightforward and matter of fact about it.

Explain to her that you and her dad aren't angry at her, and apologise for frightening her. If she asks why you were yelling at her dad maybe tell her that it was because he should've been more careful to not have those pictures where she could find them - the same as other stuff that's for grown ups gets put away or put up high (medicines, alcohol etc).

Wouldn't worry too much about her experimenting etc, she may ask a few questions but distraction works a treat to get her mind onto something else more suitable to her age - I'd be willing to bet she's far from the first kid to find dad's stash by accident