Show Media Preview Image on Views
Figured it out: visit admin/config/media/file-types and pick the file type you want to edit (Youtube media are Videos). Pick one of the file displays (Small, Big, Original) and enable the Youtube Preview Image display. Finally, pick an image style in the dropdown menu.

Then, visit the Manage Display tab. If you previously chose the Small file display, for example, edit the Display to make sure the File will be shown.