Depending on the options you pick, Facebook can automatically upload new or existing photos from your Smartphone to a private photo folder on your Facebook account.
Options include enable or disable sync; sync over WiFi only or WiFi and cellular, etc. And the app only syncs when your smartphone has proper battery level.
Each user has an allowance of 2GB for this sync private folder, from which you can select photo you want to share on your timeline.
To turn this feature on, in your mobile app, click on your Photos thumbnail located under the cover photo, click on Sync at bottom right, hit the padlock Sync Photos blue bar. Options are available by clicking the Crank icon located on top right.
Once sync’ed up you can delete photos as needed on the Facebook folder without deleting photos from your camera roll.