how do you install a seagate portable hard drive on iMac to copy photos off your pc

To reformat the external drive for macOS, see > Erase and reformat a storage device in Disk Utility on Mac - Apple Support

What photos are you wanting to move to the external drive?

If they are in the Photos Application, see > Move your Photos library to save space on your Mac - Apple Support

You should format the portable HDD as FAT or ExFAT. You need one of those formats in order to copy the files from the Windows PC to the portable HDD. Your iMac can also read disks formatted that way.

I'd suggest it may be better to format the portable HDD on your Windows PC since you are going to copy the files from that pc anyway; but you can also format the portable HDD using Disk Utility on your iMac. User den.thed already gave you a link to the documentation for formatting an HDD using Disk Utility.