how do i transfer photos from windows 8.1 to my iphone8

Follow the steps here: link-> Sync photos in iTunes on PC with devices - Apple Support

The easiest way is to install iCloud for Windows on your PC, then copy the photos to it. Then (or first) on your phone turn on Photos in Settings/[your name]/iCloud.

The next easiest way is to log on to https://icloud.com/photos and upload the photos there. Then (or first) on your phone turn on Photos in Settings/[your name]/iCloud.

You can also upload them using iTunes, but that is a deprecated method that is much harder to manage and sync.