More often than not while recovering digital photo files, typically off a memory card, the small image previews, called thumbnails, are recovered successfully, while high-resolution images themselves are damaged.
This behavior is explained by "file fragmentation". The fragmentation is said to occur when the file is stored on the disk in more than one non-contiguous blocks. The sketch explaining the process can be found at the Photo Recovery Limitation page.
The FAT or FAT32 filesystem is most likely to be used on a memory stick or a thumb drive.
Once the file gets erased on the FAT filesystem, the only information that is still available is how to locate the 1st fragment. Any subsequent blocks cannot be recovered.
Thumbnail is not large and placed at the beginning of the file, therefore the thumbnail is usually extracted OK, and still the high resolution image is garbled. Of all the data recovery types, this limitation affects mostly digital photo recovery and unformat on FAT.
