Some quick tips:
The preview image needs to be of JPG format and it's better to use the ksPreviewBuilder tool in ...\Steam\SteamApps\common\assettocorsa\sdk\dev\ksPreviewBuilder to make the previews (you may need to download the Assetto Corsa SDK in the Tools section of your Steam library first), it automatically scales it down for the launcher if you want (for better performance/load times). Just follow the instructions there.
As for the ui_skin.json it's a file that tells the game the skin name, country, team name, car number, etc. For example:
{
"skinname": "#5 Mach MAKERS GTNET Motor Sports, Super Taikyu 2016",
"country": "Japan",
"drivername": "Shirai/Hoshino/Fujinami",
"team": "GTNET Motor Sports",
"number": "5"
}
If you want you can use this:
https://1drv.ms/f/s!ApJPLIdIgk6ihIZHBn81ems_L7qFQw
Good job with the livery.
