@Astronaut
its in the comment section of the Magnet shotgun V2
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2774067269

When you decide you want to upload a vehicle in-game, press the generate thumbnail button.
In the IODV folder (%USERPROFILE%\AppData\LocalLow\Radiangames\Instruments), an image named "ws_Thumb.png" will appear.
Replace that with an image under 1mb (best if square) before pressing the upload button.
Voila.

How did you attach the photo to the thumbnail?