Edit wp default gallery

Hello all, i want to create a gallery section with a shortcode like [gallery]. I succesfully create a gallery but according to my html i need to add some more div on hover effect so i need to edit gallery. Is it possible? If so please help me out.
Example http://www.apprs.com/noruw/portfolio/
upper gallery is with gallery short code and lower one is handcoded. I want upper gallery to be looked like same with the below one.