Edit thumbnail? (CLOSED) (CLOSED)

Is there any way to change the thumbnail? I'm not seeing an option for that on the "edit" page. 

It's not straightforward - but yes, you can. 

All you need to do is click the "Edit Item" button for the post and choose a new main image. That brings up the thumbnail edit tool. 
It can be the same image if you're simply trying to edit your current thumbnail.