I found that there were no actual thumbnail levels designed for the new thumbnail style, so I made one.
Make sure you have turned your settings up to high (except for HDR), and use the new thumbnail generator. The thumbnails will be located in your Documents\BeamNG.drive\vehicles file.
Comparison: (and yes they really are that similar, I'm very pleased with this!)
Beta Photoroom Light 1.1
A white room ready for taking white thumbnails!