To continue the recovery process you have to use a PC or Laptop.
Step 1: Download Recovery Software
In this step you will be asked to enter your name and your email.
You will get an email with Download links for Win or Mac.
Download and install the recovery software.
Step 2: Scan Your Device
Plug your HTC Desire 820 to your computer.
Open Stellar Photo Recovery and scan your device.
Step 3: Recover Lost Photos
Select the photos you would like to recover.
Hit the recovery button and enjoy!