Download Steam Workshop Uploader:
https://github.com/nihilocrat/SteamWorkshopUploader



Open steam_appid.txt in the SteamWorkshopUploader folder

change the "480" to "2186320"

replace the text that is inside the config.json file with this:

Save both files, CTRL + S

Open Steam Workshop Uploader, enter the name of your workshop item and click "Create New".



Now close Steam Workshop Uploader.

Go into SteamWorkshopUploader > WorkshopContent

Find < [the name you created].workshop.json > and change the numbers next to "publishedfileid" to the workshop item's id, you can find this in the workshop page link.

Change the file name in the field "previewfile" to the image name with the file type. EXAMPLE: "Democracy_Preview.jpg" MAKE SURE THE IMAGE IS IN "WorkshopContent"
Rectangular workshop preview dimensions are 637x358

Visibility field: 0 = Public, 1 = Friends Only, 2 = Private/Hidden
Title and Description fields: copy and pasted from your workshop item.
Tags field: any of the AOC tags:
Once you've edited everything save the workshop.json file.

Now go to: AppData\LocalLow\JoySparkGames\Ages of Conflict\Custom Scenarios\THE_NAME_OF_YOUR_WORKSHOP_ITEM
If doing a map, go into "Custom Maps" instead and copy the map file into your corresponding workshop item folder in "WorkshopContent"

Now copy the contents in your Scenario folder and paste it into the corresponding workshop item folder in "WorkshopContent"

Re-open Steam Workshop Uploader and click on your workshop item on the side and click "Submit [name of workshop item]"



Done! you should now have a better looking workshop preview thumbnail.

If you have any problems or questions I will try to respond with my limited knowledge.

If you are struggling with this guide then try this one: (it's for a different game so make sure to set the app id for Ages of Conflict and configure the tags in config.json)
https://docs.studio-397.com/users-guide/uploading-content-to-steam-workshop

If you update the workshop item through in-game or through another service, you will also reset the workshop preview you applied. To fix this, you can copy and paste the updated scenario/map files to the corresponding workshop item folder in the WorkshopContent folder, then update/submit the workshop item through the Steam Workshop Uploader.