Open the Photos app
Tap Library or Albums
Select the photo or photos you want to delete
Tap the Trash icon
Tap Delete Photo or Delete [number] Photos
Open the Recently Deleted album
Tap Select
Tap Delete All or choose photos and tap Delete
Confirm the deletion
To delete photos from iCloud as well, make sure iCloud Photos is turned on
To delete photos from a computer, use Finder, Photos, or iCloud.com and remove the photos there