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: http://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() ?
