Youtube thumbnails of the following widths and heights are guaranteed to exist.
Width | Height | URL
------|--------|----
120 | 90 | https://i.ytimg.com/vi/<VIDEO ID>/1.jpg
120 | 90 | https://i.ytimg.com/vi/<VIDEO ID>/2.jpg
120 | 90 | https://i.ytimg.com/vi/<VIDEO ID>/3.jpg
120 | 90 | https://i.ytimg.com/vi/<VIDEO ID>/default.jpg
320 | 180 | https://i.ytimg.com/vi/<VIDEO ID>/mq1.jpg
320 | 180 | https://i.ytimg.com/vi/<VIDEO ID>/mq2.jpg
320 | 180 | https://i.ytimg.com/vi/<VIDEO ID>/mq3.jpg
320 | 180 | https://i.ytimg.com/vi/<VIDEO ID>/mqdefault.jpg
480 | 360 | https://i.ytimg.com/vi/<VIDEO ID>/0.jpg
480 | 360 | https://i.ytimg.com/vi/<VIDEO ID>/hq1.jpg
480 | 360 | https://i.ytimg.com/vi/<VIDEO ID>/hq2.jpg
480 | 360 | https://i.ytimg.com/vi/<VIDEO ID>/hq3.jpg
480 | 360 | https://i.ytimg.com/vi/<VIDEO ID>/hqdefault.jpg
SOURCE: Stack Overflow; special thanks to Ali Han, the author.