Unless you signed a non-compete contract and it explicitly says they own any work of yours/you cannot work on other projects, then they cannot enforce it nor can they use any of your images without your consent. They’d need to pay for licensing to do that. Especially if it’s on your time, your equipment, your art direction, editing, etc. That photo is legally yours.

I also understand the income issue…if you can’t afford to lose the income, then pick your battles, especially if it’s an at-will state. If it isn’t, then keep everything documented and maybe consider getting your work copywritten so that even if, for whatever reason, things went south, they can’t keep or use your images once you leave, they’d need to buy the rights or a license. Always do written communication for the same reason. Your boss sounds…very insecure, to put it nicely and i definitely would NOT trust them to take a moral or ethical approach if it came down to it.

Protect yourself and your work. It obviously has value.