How to move a photo from iMac to iPhone

Usually either via cable sync, or using iCloud Photos to sync the photos via iCloud.

Details:

Intro to syncing your Mac and your devices

https://support.apple.com/guide/mac-help/syncing-overview-mchl923c1147/26/mac/26

Sync content between your Mac and iPhone, iPad, or iPod touch

https://support.apple.com/guide/mac-help/sync-music-books-and-more-between-devices-mchlde9a31f1/mac

If you just want a few pictures at a time transferred, then an ideal method is to use AirDrop.

Use AirDrop on your iPhone or iPad

On your iPhone you will need to go to Settings>General>AirDrop and turn that on so that the phone will appear as one of the choices for receiving the picture. Then, almost anywhere on a Mac you can right-click a picture, choose AirDrop, and see a list of devices that can receive it. Choose your phone and, wham, it's done!

AirDrop works the other way, too, so I use it all the time to quickly get a picture to my Mac.

If you want to transfer all your pictures, then as MrHoffman said, the surest way is using iCloud. iCloud Photos transfers not just the picture you see, but also the original and all the editing steps so that you get all the information from the phone.

You can also use the older method with a USB cable and Finder, but to me that's rather like trying to send text over a land line.