Delete Photos in Photos.app
When you delete photos they are added to a Recently Deleted album where they persist for 30 days. Go to that album and you can delete them now to free up space.
Select the recently deleted album
Recently Deleted Album View
Here you can see and select recently deleted items.
Select Items to act on
Select Photos to Delete or Recover
Once photos are selected (blue checkbox) you can then recover or delete them permanently.