first things first, you'll need a contraption that you saved.
then you'll need to get into the game folder

(steam\steamapps\common\People Playground\Contraptions)

find the contraption that you just saved, open the folder and edit the image file
(i use paintdotnet for everything, even the 3d illustrations)

once you are done, you have to save the image (do not do "save as")

open the game, the icon is now finnaly shanged
and when you would want upload it to the workshop, the thumbnaill is going to be edited

i found it by myself pretty fast, you can use the same thing with other games
like gmod, where i found this method

also, the folders with numbers instead of thier names are actually your workshop subscritptions

Thx for the help ! <3

But oh wait my creation is already uploaded and it has some subscribers....

And also Wich kind of files are "accepted" to be a thumbnail?

Originally posted by Ducky:

And also Wich kind of files are "accepted" to be a thumbnail?

honestly idk, but i always save them as png. I'm sure jpg works and i want to test gif in the next contraption

Anyways thx for the Infos !