Transferring photos from iMac to new iPad Pro

Perhaps one of the easiest methods of sharing photos between your iMac and iPad is to use iCloud:

Set up and use iCloud Photos – Apple Support

The alternative is sync photos using iTunes:

Transfer files between iPad and your computer - Apple Support

You can find the iPad in Finder, click on Photos and then you can select the albums, photos that you want on the iPad. Make your selections and then click sync.