If you are looking at the in Google Photos, you might be looking at the library you have stored in the cloud. Go in the menu and select device folders. You should see the locally stored photos there. In there, if you select a picture, look in the three dot menu next to the trash bin. There should be a 'delete from device' option. That will leave the cloud stored copy alone.
If that makes you nervous, you can always select 'Free up space' from the main menu. That automatically goes through any locally indexed pictures and deletes anything that has been backed up to Google photos, leaving the backed up copy alone.