Each time you upload a video, YouTube
will automatically generate a set of three thumbnails from which to choose. You can stick with YouTube's default options or upload a custom thumbnail, preferably a beautiful photo that you have prepared beforehand.
Sign in to your YouTube channel and open the Creator Studio
> Video Manager
Select the desired video and then click on the Edit
button > Info and Settings
Click on the Custom Thumbnail
button and browse for desired image:
Use an image or photo that has been optimized from the web, preferably in the JPEG
format. The maximum file size is 2MB
Once done, click on Save changes
to apply the new thumbnail.