I'm currently finishing up a set of configurations for the Randolph Dry Van, and I'm at the point where the last thing I need is thumbnails for the vehicle selector. Since I want to replicate the official ones as close as possible, I've gone to Grid Small Pure and ticked the "Stay on Current Map" option. Despite cycling through the various configurations, the system does not appear to take pictures, and there are no thumbnails saved under the "Vehicles->Dryvan" folder.
If the thumbnail generator doesn't work you'll need to manually take pictures and name the pictures to match the trailer. Try though to make the pictures look as official as you can, but nobody is perfect