Preview Images cannot fit properly in the theme


I am owner of and i have made my site a social group and using Kaltura server, but i have a problem that is the preview of Images and videos cannot fit on theme and it overlaps on the right side widget, which make the site ugly look and i want the solution for this, is anyone guide me how to set the width/height of preview images that can be fit inside the body of my theme.
Waiting for your responses
Mohsin Malik
[email protected]