Delete backed up photos on Samsung S4 mini
If there's no option to delete, then they're already gone from your device. Google keeps a thumbnail available for you so that you can see the photo, share, etc., without taking up a ton of storage on your phone.

If you want them really gone, you'll need to go to Google Photos and delete them from there. (Actually, they'll go into the Trash, where they'll be auto-deleted in 3 days, but they'll get removed from your photo gallery on your phone.)