Welcome to Android Central! Are you concerned about storage space, or do you want low-res pics to send via messaging?
If you're concerned about storage, then take advantage of Google Photos backup: https://support.google.com/photos/answer/6193313?co=GENIE.Platform=Android&hl=en. If you upload using High Quality (not Original Quality), you get unlimited cloud storage of your photos. Use the Free Up
Space feature regularly to keep your phone's storage free of photos, since it will delete any local photos as long as they're backed up to Google Photos.