I, of course, didn't just come up with the knowledge of changing workshop thumbnails out of the blue, instead, I searched on the internet how to change the thumbnail!

Below is the link to the video that is the tutorial that I followed when I first started out (shoutout to the person who made it "Jotrain Gamer" on YT, go show some support):

https://www.youtube.com/watch?v=rzBHex0NA_U
If you want, I suggest to go ahead and watch the video, perhaps it might explain it even better than me, the rest of the guide is just me explaining my process, since it took me a few tries to understand how it works the first time I tried.

Before you start

• Make sure your Anti-Malware software lets you use the "SteamChangePreview", although personally I only use Microsoft Defender, when I had a random Anti-Malware app (whose name I can't remember) it thought it was a virus and so it wasn't working properly.
  
  
• Make sure your workshop item has been uploaded for a few minutes first, it has something to do with steam validating files and such and so you need just a little patience!
  
  
• Make sure to download your workshop vehicle, I will reference this later so keep this in mind.
  
  
• Make sure your thumbnail or preview image is 1:1 in terms of width/height ratio, a .jpg, also (and I haven't confirmed this so please take it with a grain of salt) you may need to name the file "preview.jpg" (but may you not need to name it that I'm not exactly sure).

As said in the video, go to the github page (in the video's description) and download the "SteamChangePreview" app thingy.

Click on the "Releases".








Then Click on zip file to download it.
















Put it in a folder of your choosing. Make sure to unzip it! It should look about like this (keep in mind my systems language is greek and your's probably not):

So, you have your "thumbnail", you have uploaded your vehicle (and are subscribed to it), you have the app thingy...

time to get started! (these are mostly shown in the video if you prefer a video explanation on these next steps, however, these are the steps I take so it's not exactly the same)

1. Open your file explorer 3 times so it's easier to move the images around and see what you're doing and go to:
   a) the folders with the "SteamChangePreview"
   b) Your "thumbnail" image
   c) Your workshop item/vehicle
   (if you're having difficulty with the last one go to "C:/Program Files (x86)/Steam/SteamApps/workshop/content/1532200/your workshop item's id number (you can see it on the top left of Steam when visiting its workshop page)
   
   example of how one of my uploaded item's look in the files (keep in mind mine is in greek, your's isn't):
   
   
   
   
   
   
   
   
   
   
   example of how my screen looks when all 3 windows are open
   
   
   
   
   
   
   
   
   
   
   
2. Now copy the "thumbnail" image to the workshop folder (very important)
   
   And if you want that image of your vehicle as shown from the top down perspective you can also copy that to the image folder for safekeeping!
   
   here's how that looks like
   
   
   
   
   
   
   
   
3. Now, drag and drop the thumbnail image to the "SteamChangePreview" IMPORTANT NOTE make sure you drag and drop from the workshop item's folder, or else it won't work!
   
   like this (keep in mind your's isn't in greek),
   
   
   
   
   
   
   
   
   
   After that the app thingy should open, which brings us to our next step...
   
   
4. Do as the app thingy guides...
   
   
   
   
   
   
   
   
   
   ... but be not afraid! for it is only asking the id of your published vehicle, if you've opened the file explorer to your vehicle, then the "published file id" should be the last number in that folder's file path. Else, go to your vehicle's workshop page and to the url link on the top left.
   
   example of where you can see it:
   
   
   
   
   
   
   
   
   
   
   type it in and press enter!
   
   Now, it asks for the app id, which in Mars First Logistics' case is 1532200, copy it, or if you're planning on doing this on an other game, you can see it from the store page, similarly to the published file id.
   
   it should look like this, press enter again!
   
   
   
   
   
   
   
   
   We have liftoff!
   If you see this, then you're done!
   
   
   

If in any case you see an error, then before pressing "ok" read the message bit, in my experience I have had 3 different types, as far as I remember:

An actual error I got when trying to make this guide!











If you see this then don't fret, just do as I said above:
- drag and drop the "thumbnail" image NOT FROM ANY RANDOM FOLDER, but specifically the workshop item's folder!



Other error types, I've seen (but can't replicate right now) are:

An error due to Anti-Malware software blocking the app "SteamChangePreview", just disable your antimalware or something I'm no expert I just play games!

An error because the workshop item's file had not been verified yet and so it couldn't change the preview image, there is only one solution for this and it's to wait for a few minutes before trying again, like for real, chill out! no need to instantly change the thumbnail!

Thanks for taking your time to read my guide, I hope it helped someone trying to figure out how to change thumbnails for their workshop creations!