I want to darken my image on mouse over

Hello guys,

I hope you are doing great!

I want to add a hover darken effect on the three images in this link: http://expedicionextrema.gt/inicio/

Similar to the hover effect in this template: http://www.templatemonster.com/es/demo/51191.html

I can only use CSS or JS, but I am not really sure how I can do it.

I hope you guys can help by pointing in the right direction :wink:

Thank you!