How do I get my (26F) boyfriend (27M) to realize this isn't a big deal? (Accidental nip slip)

I just got a new tattoo on my sternum. My friend is about to get a new tattoo as we speak, and she sent me a picture of her stencil. I remember I told her I'd send her pictures of my tattoo, so I went into the bathroom and snapped a quick picture on my little flip phone. It goes up in between my chest, so I had to pull my shirt up so she could see the whole thing. I didn't realize my nipple got in the picture, waaaay up in the corner. My phone screen is small and the picture quality isn't great, so I just didn't notice. I sent it to her.
I then sent it to my boyfriend, because I thought he'd like to see it. He started joking about how he loves my boobs and thanks for including that in the picture (winky face) etc. I said oh ****, I didn't realize my nipple was in there. And he said, "Seriously?" and he asked if I had sent it to anyone else. I explained that I had sent it to my female friend. He got angry and told me that he told me to be careful about this kind of thing (I never send pictures, and I was really careful when getting the tattoo to stay covered, and even used a female artist) and anyway he's ******. He told me I'm lucky it wasn't a dude or he would be really, really angry.
I've texted her about it and she has deleted the photo.
It was an honest mistake, but he has texted me like ten times and is mad and says I don't respect him. How do I get him to realize this isn't a big deal? If he sent a photo of his **** to his male friend on purpose I wouldn't care at all. And this was an accident.
TL;DR I accidentally sent a photo with my nipple in it to a female friend. Now he's ******. What do I do?


Source.