Thanks for your help I was able to use the tutorial and do it myself

Hi, it is not that easy as the most people would expect. You need to download a little steam command line program and then change the steam preview image by using it with a command for a build-in steam app. Here is a link where you can read how to use. The user ALVAROPING1 has made a nice advices how to do:
https://steamhost.cn/steamcommunity_com/workshop/discussions/search/?gidtopic=3199243752756058391&q=thumbnail&appid=585420
This Steam app only will change the preview picture that you can see in the steam workshop, not the blueprint itself!
And btw. never try to change the thumbnail-picture directly in the blueprint file of your build. This will corrupt the file for sure.

How do you make it have pictures like that while looking at it in the trailmakers workshop