Q: Deleting individual photos

If they were placed on your iPhone by using iTunes to sync them from a computer then the only way to delete them is to sync again but change the selection of photos to be synced. Each time you sync with iTunes it sets the synced photos on the phone to match the current settings/selections in iTunes. To delete one photo from the phone you can either change the selection in iTunes so that it excludes that photo or you can leave the selection in iTunes the way it was on the prior sync but move the photo to a different location on your Mac so that it isn't included in the selection.

One way of using iTunes with iPhoto or Photos that I found convenient was to create an album (or a number of albums inside a folder). Name the album or folder something like "iPhone" and then just add/remove photos to that album or folder of albums as desired. Using albums doesn't duplicate the storage space require in iPhoto or Photos and you can have the same photo in multiple albums. It makes it very easy to control just what you want on your phone.

More details on syncing to delete photos on the phone in the quote below from Sync photos from your computer to your iOS device using iTunes - Apple Support

1. In iTunes, locate and click on your device.
2. Click Photos.
3. Choose "Selected albums" and deselect the albums or collections you want to delete.
   To delete all synced photos, deselect "Sync Photos from" and, when asked, click "Remove photos."
4. Click Apply.

Learn how to delete photos and video synced from iTunes when you can't access the originals.