All below screenshots taken with my showroom and Content Manager.
Download the mod from this page, extract it and copy the showrooms to "\Assetto Corsa\content\showroom\" and the presets to "C:\Users\<YOUR_USERNAME>\AppData\Local\AcTools Content Manager\Presets\Custom Previews\".
This showroom only works with Content Manager. Do not use it with the original Kunos showroom launcher because it will look like crap. If you don't have Content Manager download it from here http://www.assettocorsa.net/forum/index.php?threads/content-manager-aka-alternative-launcher.33824/ or from here http://acstuff.ru/app/
Once in Content Manager, select a car, open one of my presets and take a screenshot. That's it!
Open Content Manager, go to content and select a car. Then click Update previews>>Options.
Then click "Switch To CM Showroom" if you have not done yet.
Then go to scene and select a preset or a showroom and take a screenshot by clicking the [+] icon.
Remember to have the latest version of Content Manager, version 0.8.882.17282 or higher, by enabling "Update to non-tested versions" option in Settings>Content manager>General.