adding CSS class to an image

I recently wiped my staging site (I’m revamping an existing site) and started over again since there had been so much time that had lapsed that I forgot how to do what I was doing. Unfortunately, I neglected to take notes on the things that I would want to duplicate, thinking I would be able to sort it out…and I am finding things I remember vaguely doing before and not remembering how. So I’m trying to recreate the best I can.

What I’m looking for is a way to add a CSS class to an image easily. I know this was done super easy before with a field in media or maybe Elementor…can’t remember… Probably a plugin or something like that. I remember it being super easy. Not having to add any code. Although it’s fine if I have to add code…I just know there’s an easy way to do this. I just cannot for the life of me remember how. And googling hasn’t turned up anything yet.

So if I want to quickly and easily add a CSS class to an image (I did, luckily, save the custom code to do what I’m trying to do…I just need to add these classes to make it work)….HOW would I do this?

Thank you in advance for any help you can give me!