https://www.youtube.com/watch?v=Ova01_MKzj8
The default thumbnail image (ie. one of 1.jpg, 2.jpg, 3.jpg) is:
All of the above urls are available over https too. Just change http to https in any of the above urls. Additionally, the slightly shorter hostname i3.ytimg.com works in place of img.youtube.com in the example urls above.