Some JPEG format images created by digital cameras (and possibly some graphics programs) have embedded information which may include a thumbnail version of the image itself. Some graphics programs like Photoshop do not always remove or update this information.
Using PHP's built-in functions that can be used for these EXIF extensions one may view this thumbnail. It is very interesting to see because the thumbnail is occasionally not in sync with the main image
To use this checker, choose a JPG file from your computer and click on the "Check!" button. If there is a thumbnail in the file it will appear below.