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.
How To Add a Custom Thumbnail to a YouTube Video
Sign in to your YouTube channel and open the Creator Studio > Video Manager > Videos:
Select the desired video and then click on the Edit button > Info and Settings tab:
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 or PNG format. The maximum file size is 2MB.
Once done, click on Save changes to apply the new thumbnail.