YouTube is owned by Google and Google likes to have a reasonable number of images for different screen sizes, hence its images are stored in different sizes. Here is an example of how your thumbnail will be like:

Low quality thumbnail:

http://img.youtube.com/vi/<YouTube_Video_ID_HERE>/sddefault.jpg
Medium quality thumbnail:

http://img.youtube.com/vi/<YouTube_Video_ID_HERE>/mqdefault.jpg
High quality thumbnail:

http://img.youtube.com/vi/<YouTube_Video_ID_HERE>/hqdefault.jpg
Maximum quality thumbnail:

http://img.youtube.com/vi/<YouTube_Video_ID_HERE>/maxresdefault.jpg