Actually, the lost photos are still in the memory space, you can recover lost photo by some Samsung Photo Rcovery software. But the first thing you have to do is stopping using the phone, and follow the steps:
Step1. Download the Sasmung Photo Recovery from here and install it on your computer.
Step2. Connect your Samsung phone to computer via USB cable or connect the SD card in the phone with card reader, and launch the Sasmung Photo Recovery application.
Step3. Click to choose the drive that means your Samsung galaxy phone and Select the option "Photo Recovery".
Step4. After scanning, you can preview the files and Choose the ones you want to get back and click "Recover".
Step5. "Save" all your recovered photos in the Samsung galaxy phone and you have finished Samsung galaxy photos recovery.