Suppose that you want to do a screenshot of the following video:
https://www.youtube.com/watch?v=rCq6A6ZHHAc
There is a little trick:
So your screenshot will be:
http://i.ytimg.com/vi/rCq6A6ZHHAc/0.jpg (screenshot)
if you want a thumbnail you can write the following:
http://i.ytimg.com/vi/rCq6A6ZHHAc/1.jpg (thumbnail 1)
http://i.ytimg.com/vi/rCq6A6ZHHAc/2.jpg (thumbnail 2)
http://i.ytimg.com/vi/rCq6A6ZHHAc/3.jpg (thumbnail 3)
the default thumbnail is the following:
http://i.ytimg.com/vi/rCq6A6ZHHAc/default.jpg
For the high quality version of the thumbnail use a url similar to this
http://i.ytimg.com/vi/rCq6A6ZHHAc/hqdefault.jpg
for a medium quality version of the thumbnail, using a url similar to the HQ:
http://i.ytimg.com/vi/rCq6A6ZHHAc/mqdefault.jpg
For the maximum resolution version of the thumbnail use a url similar to this:
http://i.ytimg.com/vi/rCq6A6ZHHAc/maxresdefault.jpg
