Apple made it extremely easy to recover deleted photos in its latest OS.
This is how to do it:
1: Access your Photos App and go into your albums.
2: Scroll to the bottom and tap on "Recently Deleted."
3: In that photo folder you will find all the photos you have deleted within the last 30 days. To recover you simply have to tap the photo you want and press "Recover." You can also press "Delete" to get rid of that photo permanently.
Note: If there's a picture you were looking for that's older than 30 days then you would have to either restore your phone using a saved backup on iTunes or used a third-party photo recovery app like easeus data recovery.
If your phone is Android:
Download the easeus data recovery wizard for android.