Disable opacity on portfolio items

Hi,

First of all, fantastic theme .. I love it although maybe, the documentation could have been more detailed.

What I would appreciate is your help in a small matter I am having as i am a total newbie, lol. I am trying to do the image cover of the portfolio opaque, with the picture not visible when not hovered over. I attached the pic, to show you better .

Can you please guide me how this can be done?