It often happens that while doing the digital photo recovery, typically off an SD card, the small image previews (called thumbnails) are recovered OK, but full size images are damaged. This behavior is explained by so called "file fragmentation". The fragmentation is said to occur if the file is stored on the disk in multiple non-contiguous blocks. The sketch explaining the fragmentation can be seen at the Photo Recovery Limitations page.
In most cases memory cards use FAT or FAT32 and when the file is deleted on the FAT filesystem, only the information about the 1st fragment remains. The second and the following data blocks cannot be recovered. Thumbnail image is usually small and it is placed near to the very start of the file, hence it can be often extracted OK, while the high resolution photo will be lost.
Of all the data recovery types, fragmentation mostly affects digital photo recovery and unformat on FAT filesystems. Using the file recovery utilities in this case is almost always useless, for there is no needed data in the restored file.