I found this guy called "TechnologicNick" who made a program for Steam that allows you to change the Steam Preview image of your workshop item. It looks a bit scary at first but trust me it's not as bad as it looks.

This is the link to the page with the program
https://github.com/TechnologicNick/SteamChangePreview

This is a link to the video that shows you how to download the program and it shows you how to use it.
https://youtu.be/rzBHex0NA_U?si=krxa1AfQCTd0TTTb

Be sure to watch the video as it gives a lot of insight of how this program works. Thanks for reading.