You can easily view (and download) the thumbnail of any Youtube video in high or default resolution by searching for the OG image tag in the HTML source page of the video.
Here are the steps:
Step 1: Open the youtube video in your browser. For this example, I am using the following video:
Note: I am using Firefox as my browser, so the instructions here are specific to Firefox, but this works in Chrome and Internet Explorer as well.
Step 2: Press CTRL + U on your keyboard to open the HTML source of the page.
Step 4: On the HTML source page, do a search (CTRl + F) for the phrase 'og:image'.
Now you should be able to see the thumbnail URL as shown in the image below:
From this tag, the URL of the thumbnail image would be: https://i.ytimg.com/vi/mcixldqDIEQ/maxresdefault.jpg
You can now copy and paste this URL in your browser to view it and then download it if you want.
Step 5: To download the image just right click on the image and click 'Save Image As' and then 'Save' to save it to your computer.
Method 2: Viewing the thumbnail by using the video ID
If the above method does not work, you can use this instead:
Step 1: Copy the 11 character alphanumerical ID of the youtube video (that you wish to view the thumbnail for).
For example, the ID of the following video is: 8hYlB38asDY
Step 2: Paste the ID where it says ID in the following URLs:
Step 3: Open these URLs in your browser to see the thumbnail image. You can then download the thumbnail by right clicking and saving the image to your computer.
Note that hqdefault.jpg will give you the standard definition image and maxresdefault.jpg will give max resolution image. Also note that not all videos will have max resolution images, but all will have standard definition images. If the video does not have a max resolution image, you will get a 404 error for the maxresdefault.jpg URL.
This method can be a little confusing as youtube has obfuscated its HTML code, but if you want to try it, here's how:
Step 1: Open the youtube video (that you want to see the thumbnail for) in your browser.
Step 2: View the HTML source of the video by clicking 'CTRL + U' or 'CMD + U' if you are on a mac. You can also view the source by right clicking on a black area of the screen and then selecting 'View Source'.
Step 3: Once on the HTML source page, click 'CTRL + F' to bring out your browser's search box and search for the term 'hqdefault.jpg'.
Step 4: Once found, copy the entire URL starting from 'HTTPS' and ending in 'hqdefault.jpg'. The URL should look something as follows:
Step 5: Remove all the back-slashes from the URL, so it looks as follows:
This is the URL of the standard definition thumbnail.
Step 6: To find the URL of higher resolution thumbnail, search for 'maxresdefault.jpg' and repeat the above steps. Note that not all videos will have a max resolution thumb.