1.Login to your blogger dashboard--> Design- -> Edit HTML
2.Scroll down to where you see ]]></b:skin> tag .
3.Copy below code and paste it just After ]]></b:skin> tag
<style type='text/css'>
.PopularPosts ul li {background: none repeat scroll 0 0 transparent;float: left;list-style: none outside none;margin: 5px !important;padding: 0 !important;}
.PopularPosts ul li img {padding:0;-moz-border-radius: 5px;-webkit-border-radius: 5px;border-radius: 5px;-webkit-transition: all 0.3s ease;-moz-transition: all 0.3s ease;transition: all 0.3s ease;border: 2px solid #CCC;height: 80px;width: 80px;}
.PopularPosts ul li img:hover {border:2px solid #BBB;-moz-transform: scale(1.2) rotate(-1090deg) ;-webkit-transform: scale(1.2) rotate(-1090deg) ;-o-transform: scale(1.2) rotate(-1090deg) ;-ms-transform: scale(1.2) rotate(-1090deg) ;transform: scale(1.2) rotate(-1090deg) ;}
</style>
Note: Please You first Add Popular Posts widget to Your Blogger Sidebar. (Blogger widgets list..)4 Now Click on Save Template,
5.Now Set Settings Your Popular Posts Widget , Like this,
6. Now Save Your Popular Posts Widget and You are Done..
Cheers.