Portfolio ThUmbnail Image

I have changed portfolio section to something else. Now, what I want is as follows:

I don’t want to show image on my portfolio page i.e.

http://globalsqa.com/portfolio_item/selenium-online-training/

I just want to show image in its thumbnail when we show it at home page or at following page:

http://globalsqa.com/portfolio/

What can be done to fix this?