Youtube video thumbnails are available in four different qualities and dimensions.

  • SD 640 x 480
  • MQ 320 x 180
  • HQ 480 x 360
  • HD 1920 x 1080

To download your desired size thumbnail without any tool is too easy.
There are four URL given below through which you can download your desired size thumbnail by just replacing OOOOOOOOO with youtube video id.
SD 640 x 480
MQ 320 x 180
HQ 480 x 360
HD 1920 x 1080
For example i want to download thumbnail of a youtube video what i have to do is just copy video id from url which will be look like this Fl54gG0B8I0 . In simple words copy everything after watch?v=OOOOOOOOO.
Suppose i want to download HD 1920 x 1080 size thumbnail, what i have to do is just replace Fl54gG0B8I0 with videoidhere in given URL