You can remove pictures just from the device by going to the upper left and clicking on the "Photos". That should bring up a menu, where you scroll down almost to the bottom to get to "On Device". Bringing up that list is just the photos on the device (instead of all the photos that G+ knows about), and when you delete there it only affects the device storage, not the backed up version. See https://support.google.com/plus/answer/3453521?hl=en for a (very) little more detail.