You can list thumbnail URLs by size
Youtube Thumbnailist
Youtube video thumbnails have multiple images of different sizes. Copy and use an image URL of an appropriate size, such as when posting it on a blog.
Please copy & paste the video URL 
Youtube
[default] Default thumbnail image and URL
Size: 120 x 90
[mqdefault] Medium quality thumbnail image and URL
Size: 320 x 180
[hqdefault] High quality thumbnail image and URL
Size: 480 x 360
[sddefault] Standard definition thumbnail image and URL
Size: 640 x 480
[maxresdefault] Maximum resolution thumbnail image and URL
Size: 1280 x 720
