The Popular Posts Widget is one of Blogger's official widgets and it is very easy to add this to your Blog. Here is a simple tutorial for creating beautiful Popular Posts Widgets for your Blog using CSS. No need to edit the template.
Step I: Adding the Popular Posts Widget to Your Blog
From your Blog's Dashboard, navigate to Layout --> Add a Gadget --> and select Popular Posts from the list. Under the 'Show' section, check 'Image Thumbnail' and uncheck 'Snippet'. Also select the number of posts from the drop down list. Now click Save.