How to Change Steam Workshop Thumbnail?!

Hi,

As the title says, how do i change the thumbnail for my map?

Ive looked everywhere but the only things I can find are for other games...

I have tried changing the preview PNG's under: steamapps>common>SpingtiresMudrunnerEditorBeta>Media>_mods>MyModNumber

After that I tried re-uploading the mod but the thumbnail doesnt change...

Any help appreciated!

@pearce95 Really easy. All you do is replace preview.png with whatever you want showing up instead.
If you need editing help I recommend gimp or Photoshop.