1) The phone has to be rooted. Rooting it now qould probably overwrite some of the pictures y9ou're trying to recover.
2) You need a file recovery app. Installing it now (.. see above).
3) If 1 and 2 are met, run the recovery app.
4) ALWAYS keep anything you don't want to lose backed up to at least 2 independent destinations. Any file not backed up to 2 independent destinations is a file you don't need. (And learn how cloud storage works - sync is 2-way.)