How to add a custom thumbnail for Space Engineers workshop. Go to:
C:\Users\"yourname"\AppData\Roaming\SpaceEngineers\Saves\"long numbers"\"your published world name" and find thumb.jpg and replace it with your desired picture (make sure the new picture is titled thumb as well). Then open Space Engineers go to load world, and find the world you published, then just press publish again. Your custom thumbnail should then be present on your workshop item. Hope this helps.