How can someone hack my iPhone?

no one can hack into your phone.. do you share anything with him? Like an Apple ID or a computer that may have some of your backups? Does he know the passcode to your phone?

If any of the above... you should not share Apple IDs with anyone, or your Apple ID credentials.. and if you don't trust him, he certainly shouldn't know your iPhone passcode

Change your Apple ID password - Apple Support

Use a passcode with your iPhone, iPad, or iPod touch - Apple Support

About encrypted backups on your iPhone, iPad, or iPod touch - Apple Support

If your iPhone is not jailbroken, nobody even your ex can hacked an iPhone.

He is not that smart.

Make sure you change your Apple ID password, see here => Change your Apple ID password - Apple Support

and enable 2 factor authentication, read here => Two-factor authentication for Apple ID - Apple Support

Important: Use at least 2 trusted phone numbers (landline can also be used for 2 factor authentication).

Even if he knows someone that used to work for Apple.. they don't have a 'back door' or a trick.. If they did you'd find all about it with a simple google search.

Odds are he figured out your passcode or something of the sort. Or if this picture is all the evidence there is, maybe he got it some other way.

Just to be safe and for piece of mind, you should reference both of my previous links and change your Apple ID password and change your passcode.. and also if you are backing up your iPhone to a computer, make sure it's encrypted. And as ckuan has suggested, make sure you are using 2 factor authentication

Also.. check that there are not any unknown/unexpected devices signed into your iCloud account

Check your Apple ID device list to see where you're signed in - Apple Support

It’s probably something called “gaslighting”. If so, it’s intended to scare you.

Telling you a story, and showing you something hat doesn’t actually prove access.

That photo might be because your password is known and that would be Bad (and you’ll want to change that password Right Now, if so), or because of an old backup on your iPhone on a computer that your ex now has, or because you’re still in Family Sharing and haven’t exited, or because the old photo was previously shared, or your ex still has a trusted device, or because you have re-used a password across different services and have previously been caught up in some unrelated web service breach (and again If that password has become known, you’ll want to change that Apple ID password Right Now).

Again, enabling two-factor authentication on your Apple ID is important here, as has been suggested above. This makes it harder to wrest control of your Apple ID. And it’ll flag attempts to log in.

In the US, the folks that enforce CFAA 18 USC § 1030 tend to take a dim view of unauthorized access, if unauthorized access occurred here.

And for completeness, some folks will attempt to “phish” or spoof or otherwise con somebody else into exposing their (your) password:

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

I’m not as certain of the infallibility iPhone security as some of the respondents above, but I doubt that your ex has access to GreyKey equipment or similar forensics. And that would still require physical access to your iPhone. And a million dollars is a small expenditure in the offensive computer security business.

Having an old picture that doesn't proved anything.

Even working for Apple will not know how to hacked an iPhone.

There's no trick whatsoever.

Do you know the FBI invested $1,000,000 to get in a terrorist iPhone 5 (not your modern iPhone XS Max) ?

As said your ex is not that smart.