If you want to get rid of the "black bars" and do it like YouTube does it, you can use:
https://i.ytimg.com/vi_webp/<video id>/mqdefault.webp
And if you can't use the .webp file extension you can do it like this:
https://i.ytimg.com/vi/<video id>/mqdefault.jpg
Also, if you need the unscaled version, use maxresdefault instead of mqdefault.
Note: I'm not sure about the aspect ratio if you're planning to use maxresdefault.