I own a TomTom XXL 550 GPS and have a subscription for quarterly map updates. I bought the GPS from Woot for $60 --- the map subscription costs more than the hardware.
Anyway, I get an e-mail a few days ago informing me that a new update was ready for me to download. So I connect my GPS to the Mac and run the update software, which downloads the update via the Internet and installs it in my GPS.
At DSL speeds, this is a few hour process, so while the download was in progress, I noticed that the GPS had an icon on my desktop as if it was another disk drive. So I decided to poke around and see what was stored in the GPS.
I found a folder called "Art" and in that folder was a subfolder called "Cars." I opened that folder and saw that it contained bitmapped images of the various vehicles you can choose to represent yours on the GPS. My choice was a green SUV that kind of resembles my green 2007 Subaru Outback Sport.
The nerd LED lit up over my head and I wondered if I could take a photo of my Subaru, Photoshop it so that it matches the size and format of the other images in the Cars subfolder. I took a few photos, played around with the images in Photoshop for about a half hour, and added a new image into the Cars subfolder.
After the map update was downloaded and installed, I checked to see if I could select my Subaru image from the TomTom options. My image was one of the options, so I selected it and tested it by and sure enough, my Subaru is now represented exactly by the GPS.
I can almost read the call sign on my car's license plate, but not quite!