This is from Disk Digger website
Why do some photos recovered with DiskDigger appear lower-quality or pixelated? On a non-rooted device, DiskDigger is limited to recovering images from your thumbnail caches, which means that the images will likely be lower-quality than the original photos. This limitation applies to both the Free and Pro versions of the app, and cannot be avoided. Recovering the full-resolution original versions of the photos is only possible by rooting your device. (Note: even with a rooted device, recovery of the original photos cannot be guaranteed, since it's possible for the photos to be fragmented or overwritten by new data.)
As I stated, your device needs to be rooted.