How can I delete pictures taken with iPhone?

Follow the steps here

https://support.apple.com/en-us/HT205856#:~:text=Delete%20a%20single%20photo,button%20%2C%20then%20tap%20Delete%20Photo.

Lickity_split wrote:

It will never be fully gone from the phone

This is not correct. Once deleted, the pictures will be moved to the recently deleted album in case you want to retrieve them. After 30 days they will be completely and entirely removed from the iPhone.

You can delete them from the Recently deleted album before the 30 days if you don't want to wait.

It will never be fully gone from the phone

And in addition to what Phil said above you can actually go into the Recently Deleted album and delete them right away.