C:\Users\ username(REPLACE THIS) \AppData\Local\BrickRigs\SavedRemastered\Vehicles

To get to AppData, you'll need to type %APPDATA% into the Windows search bar.
In the Vehicles folder, there should be a bunch of folders with numbers for names, that's basically a list of all your 1.0+ creations starting from the first one. Once you find the correct vehicle folder, you have to replace the file named "Preview" with your custom thumbnail.
be sure to make your custom thumbnail traditionally https://youtu.be/WWSx8Je7wrM?t=32