How to add Overlay text on thumbnails of WordPress Popular Posts Plugin?

By overlay text on thumbnails I mean this.

I want to add overlay text on thumbnails of WordPress Popular Posts Plugin. I tried adding html and css by following several tutorial available on web, but I was not able to do it.

Can someone help me to do it?

I took reference from the following tutorials:-
http://web.enavu.com/tutorials/making-image-overlay-caption-using-css/

http://blog.neatlysliced.com/2007/02/images-with-caption-overlays/

http://wordpress.org/extend/plugins/wordpress-popular-posts/