Welcome to Android Central! You could use a photo resizer app -- there are a number of them in Google Play. But if you're concerned about storage space, why not take advantage of Google Photos Auto Backup? https://support.google.com/photos/answer/6193313?co=GENIE.Platform=Android&hl=en. Once your photos are backed up to the cloud, you can then use the Free Up Space option to delete the photos from local storage, since they're already safe in the cloud.