How do I transfer photos from my PC to my iPhone?

One method is to sync them from the computer to the iPhone using iTunes and a USB cable. See the iTunes section of Keep your photos safely stored and up to date on all of your devices - Apple Support

Note that you should keep the originals on your PC as any future iTunes syncs with the phone will delete the previously synced photos on the phone if they are not included in the subsequent sync(s).

iCloud Photo Library is another method; see the above link for more information on that also.

Hi

In itunes on your pc enable iCloud share in photus & iCloud streaming

On iPhone go to settings photos & camera enable iCloud share

Make sure all icons in photos are enabled to green.

Cheers

Brian