I made this guide because this was a problem of mine and i couldn't find a way to do it on steam. It took me a while to figure out but now that I did I'd like to share it with you

First make a level or machine (for machines the save path is different but the start is the same) and save that. It will have a ugly automatically generated tumbnail. To change this you should first make the thumbnail. Any image editing software will do the job for making this will. Make sure you image is at least 195x195 but I suggest to get an 512x512 jpg file.

Once you`ve made that thumbnail overwrite the wrong one on the right location. you should go to:
ps: You can get there in the game by clicking on the thumbnail.

This Pc
Than C:
Programfiles (x86)
steam
Steamapps
Common
Besiege
Besiege data
Customlevels (or savedmachines)
And at last Thumbnails

Than you must save your image onto the right thumbnail. In photoshop make sure the saving format is png. If you did this you can go inside the game and if its right the thumbnail is applied. Than you can just upload it to the workshop with a great thumbnail. Make sure you won`t overwrite the save anymore because it will get a ugly thumbnail again.

If you uploaded your thing and did it right it will look like this:

[url:https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1267789553]

I hope this helped you getting a thumbnail on your level. I updated this last on the 25 of December to fix some issues with the guide.