He is asking about the stereo... TomTom has nothing to deal with this issue. This is for sure doable but it all ends up with the question: how many hours are you willing to put on this issue ?
You have to somehow dump the filesystem of the headunit and look for the image. Note that the splash image might be embedded in the application which can be harder to modify....
I've hacked my digitalstream PVR to show a pic of my Cx-5 on startup, much to my family's annoyance. just need to do the same when the cx-5 stereo starts up.