How to make thumbnails overlap the slider image in a gallery section

I added a gallery section in my 7.1 website and I set it to "simple slideshow" with thumbnail feature. Now I want to make the thumbnail to overlap the slider image. I have included my present gallery section and the result I want to achieve in the images below. Kindly check them out. I'll be glad if you can provide a solution.

Hi,

I think we can use some CSS negative margin to move up thumbnails 

Can you share link to page where you added gallery? We can check easier