Humminbird 788c Operation Manual page 169

You may view data files associated with each screen snapshot by removing the
MMC/SD card from your unit and installing it in your optional-purchase MMC/SD
card reader connected to a PC. Use Windows Explorer to see the contents of your
MMC/SD card.
You will notice that a .DAT (or a .TXT) file is created for every .PNG screen capture.
This data file includes the thumbnail view, and is required to view the screen
snapshot from your unit, so don't delete these files.
NOTE: For recording information, see Snapshot and Recording View and Snapshot
and Recording X-Press™ Menu.
157 Accessories Menu Tab