Youtube has made it a little difficult to view the image URL for the thumbnails by obfuscating the HTML code. But no worries, here's another method by which you can easily view the thumbnail.
Method 1: Viewing the thumbnail by using the video ID
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.
Method 2: Checking the HTML source of the video
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 forward 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.
Method 3: Older method
This is the older method which probably will not work but I am leaving it here in case you want to try it.
You can view and download the thumbnail used in a Youtube video by checking the page source and then looking for the "OG Image" Open Graph meta tag.
The "OG Image" meta tag is one of several Open Graph meta-tags that youtube uses to tell social networks like facebook which image to use in-case someone shares/embeds a video in their social network page. In 99% of cases, the "OG Image" tag that youtube uses is the thumbnail of video.
So here are the steps on how to locate the "OG Image" tag and there-by retrieve the URL of the video thumbnail:
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: Point your cursor on a blank area of the video page.
Step 3: 'Right click' on the blank area and select View Source or View page Source as shown in the image below:
Step 4: On the view 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.