Bumping this from six years ago, for an update:
Here are the links to a video's thumbnails and default image.
Thumbnails. There are 3 different ones. "default" and "2" are the same:
http://img.youtube.com/vi/<insert-youtube-video-id-here>/default.jpg
http://img.youtube.com/vi/<insert-youtube-video-id-here>/1.jpg
http://img.youtube.com/vi/<insert-youtube-video-id-here>/2.jpg
http://img.youtube.com/vi/<insert-youtube-video-id-here>/3.jpg
Default image. "0" and "hqdefault" are the same:
http://img.youtube.com/vi/<insert-youtube-video-id-here>/0.jpg
http://img.youtube.com/vi/<insert-youtube-video-id-here>/mqdefault.jpg
http://img.youtube.com/vi/<insert-youtube-video-id-here>/hqdefault.jpg
http://img.youtube.com/vi/<insert-youtube-video-id-here>/maxresdefault.jpg
However, hotlinking any of those using the img tag still produces a small thumbnail and not the bigger image.
Here's how to get bigger size thumbnails.
http://img.youtube.com/vi/<video-id>/<resolution><image>.jpg
where:
<resolution>
- lowest resolution
sd - Standard Definition
mq - Medium Quality
hq - High Quality
maxres - MAXimum RESolution
<image>
default - Default image (1, 2, 3 shot from video, or custom uploaded)
1 - First shot from video
2 - Second shot from video
3 - Third shot from video
So to get the third thumbnail in the highest resolution:
http://img.youtube.com/vi/<video-id>/maxres3.jpg
So using
this YouTube video of
Leanne Crow as an example, here's the code to produce the second thumbnail in high quality:
http://img.youtube.com/vi/tJr-jbRVeRQ/hq2.jpg
And here is the result: