Thumbnail in Widget

Sorry that is not possible because the thumbnail is not retrieve from the query to the DB.

You can do it by editing the plugin code. Look for wp-postviews/wp-postviews.php and search for the function you’ll be using in your widget.

In my case I am using most viewed pages/post. So I scrolled down and find:

### Function: Display Most Viewed Page/Post

Then you look for the last $temp line (within that function):

$temp = str_replace("%POST_TIME%", get_the_time(get_option('time_format'), $post), $temp);

And you add this right below:
$temp = str_replace("%POST_IMG%", get_the_post_thumbnail(), $temp);

Now, get back to the template and set it using %POST_IMG% wherever you want the image to appear:

<li>%POST_IMG%<br /><a href="%POST_URL%" title="%POST_TITLE%">%POST_TITLE%</a></li>

Remember to have a fallback image: https://justintadlock.com/archives/2012/07/05/how-to-define-a-default-post-thumbnail

This is actually pretty simple and could come with the plugin.

Is not that straightforward with the current code base. By using get_the_post_thumbnail(), you will incur 1 query for every post make. So if you are showing Top 20 Most Viewed Post, it will have additional 20 queries.

A better way of doing this will be to use query_post() to replace the existing widget sql query. That is on my to-do list.

Do you finish? And how to replace the existing widget sql query with query_post() ?