How do I assign a property to an image, and create a group?

I’ve created a lightbox, and want it to display a group of images, one at a time with lightbox arrows to navigate to each image in the group, in that lightbox. There’s another thread that states: it’s just a property - images using the same group name will be displayed in the same lightbox.

But I’m stuck: how do I assign a property to an image?

I have looked around, searched, no luck. It’s probably something simple, but I’m not seeing it. Hopefully with the great community here, someone will have the easy answer, and then when the next person gets stuck like I have, this thread will show up in search.

Thanks in advance.