AIBU to be upset my husband posted an unflattering birthday photo?

Horrible thing to do so YANBU.

Although if you've ever posted photos of your kids online, you're basically doing the same thing to them.

I thought it was a way of describing people popularised on social media? So I assume that people who are heavily influenced by social media and are primarily interested in superficial physical appearance will think it is a normal way to talk about people and compare them.
I think that a lot people who live in the real world find this way of viewing other human beings as offensive.

I would get the ick, can he not think for himself

Why does he have to be like his brother

Why do you have to be like his brothers wife

You’ve misunderstood. As have others. Re read the op. The ops bil amd his wife have a jokey relationship whereby he posts embarrassing photos of her. The ops husband tried to replicate this and got upset the op didn’t find it funny. It literally says this in the op, and what your dh does with a completely different intent is utterly irrelevant

I don't think it is. I don't think you will find a huge amount of people agreeing with you to be honest.

Unfortunately I have heard it very many times and it is an extremely offensive way of rating people's appearance as though they are in a cattlemarket.

Negging. Doesn't want you getting ideas above your station, and thinking you're too attractive for him.

Top tier twatty behaviour. I would do something similar to him. Then again, you're in danger of becoming The Twits.

The brother’s wife might actually mind but feels like she has to laugh along. I remember when Russell Brand posted the photo of Katy Perry and she wasn’t best pleased!

Huh? 10/10 is a common phrase used for women or men - have you never heard this before? It’s another way of saying someone is very good looking…

Then his "defence" was that his brother does the same to his wife 🙄

YANBU @SassyNavyHam Keep your boundaries in place.

It sounds like there isn't much fun in your relationship and your DH made a poor attempt at injecting some. Have you got busy lives with DC? Do your insecurities get in the way of you relaxing and having fun? Do you find him boring?
It was a poor move from him if he knows how insecure you are, but if you don't want to end the marriage maybe this is an opportunity to have a conversation about what you could both do to improve your marriage.

It all depends on your relationship, my husband does it and I laugh because I do the same to him! I don’t find it humiliating, it’s just a joke!
At the same time if you don’t have that kind of relationship then I can understand you’d be upset and rightly so.
From now on he needs to understand where the boundaries are and stick to them.

He did it because he thought it was funny. He basically said as much. He should know your relationship well enough to be aware that you would hate it. But he did it anyway. Constantly comparing you to his brother's wife is immature and insulting. Tell him to go and live with her if he feels that way.

There are two issues here.

1. Can you have a laugh? Do you need to loosen up? I think if he wants to enjoy himself and have a laugh with you, that’s sweet and you should reflect on whether you are a bit on the miserable side and why!
2. he’s really fucked up there. My DP finds it funny to take a picture of me and then will get ChatGPT to make it really horrible or funny and he will share it in our family WhatsApp. I can laugh at that because it’s not actually me and it’s not public but I would be so mad if he let everyone see a bad version of me. It’s not even his original idea and if you’re so boring and not good for a laugh he should know it’s going to rile you.

Sometimes people get defensive when they do something that doesn't land well and hurts their partner. It sounds like his brother posted a non posed photo of his partner as a 'joke' and she doesn't mind, you did mind and so your partner is on the defensive as you sent him a long message explaining why and were annoyed with him. He did delete it but he should also have apologised as it upset you.
But this has also brought up his issue that he believes his brother and his partner seem to have a more fun relationship and he wishes you both could joke more. Obviously his 'joke' wasn't your idea of a joke but is it true you don't have much fun together? Maybe this is a genuine issue for him.

I don't, so no need to be sorry. DH knows to check with me before posting anything, and I do him the same courtesy.

Well I don’t think he did. My DH sometimes wants to post photos of me that I’d rather people didn’t see. Not because he’s a dick, because he genuinely sees nothing wrong with it. Not everything’s man does is to get at the wife. I’m sorry you have men around you that makes you feel like that tbh

He sounds like he has a very immature relationship with his brother and hasn't got over the childish wanting what he has, and to be the same.
If this is going to manifest as trying to copy his brothers relationship, despite you being an entirely different woman, then this photo is the tip of the iceberg of your problems.

My ex did this, he was also abusive in other ways.

Ugh. He’s bringing his weird childhood issues with his brother that he doesn’t get on well with into his adult life. And using you on your birthday to do so. AND resenting you for calling him out on being unkind.

if it were a genuine joke that landed badly, and he loved and respected you, he wouldn’t get defensive or argue he didn’t have bad intentions. He’d care about the impact on you and be genuinely sorry for upsetting you on your birthday.

but he’s not sorry he upset you, in fact he doubled down and blamed you for his unhappiness in comparison with jealousy he feels about his brothers relationship.

would he be willing to talk it through with a third party in couples therapy?

So sorry OP. You deserve better than the nasty pratt you married.

There's an article in August's Marie Claire 'Why do Men Ruin your Birthday' ~ sorry I can't link. Have a look. Men ruining special occasions is A Thing. But we already knew this really, didn't we?

But he defended himself by saying it was a joke

how is posting a beautiful photo a joke?

No this does not fly

Referring to a woman as a 10/ 10 is reducing her to less than human.

You are being extremely harsh on yourself comparing yourself so unfavourably to her.
Does he generally try to make you feel bad about about yourself because thats what it sounds like: the type of guy who needs to try and chip away at his partners self esteem.

No, he said he wanted OP to have a sense of humour, so it doesn’t sound like it was posted with a sentiment of “look how beautiful my wife is”. Which I still wouldn’t think was ok, given she was sleeping, but would at least have a defensible sentiment behind it.