How to delete photos/videos off phone/icloud

The software needs to be updated on my phone, but there is not enough storage space available. To make more space available, I've been trying to delete photos/videos off, but they won't delete. Isn't there an option to delete specific photos/videos immediately off icloud?? I delete, but they show up again :( It says that it's going to permanently delete them off, but then they show up again. HELP!!