I found this on the Corvette Forum, posted by user: Odin 11-19-2009

"I don't know if this works on the 6140, but it should. Some of these instructions might not be the same for the 6140 if the icons and controls are different, but you'll get the idea.

On the DNX9140...

First you will want to save your settings by going to SOURCE SELECT and touching the arrow at the top right side of the screen, select the Wrench icon and touch SYSTEM, page down to the last option, SETUP MEMORY. Save your settings to recall later.

Next you will need a USB flash or thumb drive. Create a folder called "PICTURES" on the USB drive and place your image of choice in the folder. Remove the drive from your computer and follow these steps:

Turn unit on and hold down buttons: SOURCE and VOLUME DOWN, Hit the RESET button once. The unit will reboot, continue holding the SOURCE and VOLUME buttons until the screen illuminates. You can now release the buttons.

You will see the setup screen, touch OEM, at this point you can plug in your flash drive, select USB and select the name of the drive that appears.

Navigate to the PICTURES folder and select your image. Hit OK, the unit will pause for a few seconds and return to the previous screen. Hit the RESET button once more and allow the unit to boot.

You should now see your custom startup screen. Restore your saved settings.
"

Very informative.