I think deleting the photo on the phone won't delete it in the cloud, but you have to be careful how you delete it. If you use the Gallery or Photos app, it might ask you if you want to delete the photo on the phone and everywhere else--obviously, don't do it that way. If you use Photos, swipe in from the left and select On Device. Photos you delete there shouldn't affect any cloud backups, but you may want to test it on a meaningless photo first.
I would guess that if you used a file manager app to delete those photos from the actual directory, it would also achieve the same goal (and probably more easily, if there are lot of photos to delete).