Parallax images stretched way beyond screen dimensions

I feel like I’m missing something terribly obvious here but on the site I’m working on, the images I’m using in the Color Sections with the parallax setting are being stretched full width, which I understand is a part of the functionality but it’s impossible for me to have the actual imagery I’d like to use from the client show up correctly on a 15″ screen.

How do I have the photos fit to screen so that the subject of the images actually shows on the site and doesn’t get cut off at the sides? An example can be seen here: http://twintracksexpeditions.com/about-us

The image behind the photo of the men is a photo of three penguins (which are meant to show up under the text on the right) but you can only see the shoulder of one of the penguins in the bottom right corner.

Can you please point me in the right direction to tweak the settings? I’ll be using a number of photos in this manner but need them to sit correctly with the subject matter in the background, visible.

Thanks so much.