Photos

Good afternoon shirleyfromdurant,

Welcome to the Apple Support Communities!

I understand you have photos on your iMac that you want to have on your iPhone. The easiest way to accomplish this is to enable iCloud Photos on both devices. This will keep all of your photos synchronized between your computer and your iPhone. When you add, edit or delete photos, the action will be applied to all devices signed in to iCloud. The resource below explains how this works:

Set up and use iCloud Photos

You can export pictures from your Photos library on your iMac and view them using Finder on the Mac or the Files app on your iPhone. However, unless you are using a photo application, they are treated like any other data file.

Photos Help - Export photos, videos, and slideshows 

Use the Files app on your iPhone, iPad, or iPod touch

Take care.

If you don't want to use iCloud you can do it with the Finder. Do the following;

1 - connect the iPhone to your Mac and unlock it.

2 - open Photos and put the photos you want to upload to your iPhone in an album with a distinctive name so it's easy to find in a list.

3 - open a Finder window and click on your iPhone in the sidebar.

4 - click on Photos, on Selected Albums, on the album you want to sync and then on Apply:

Or you can always email them to yourself. 8^)