Making An Image Responsive On Mobile Devices

Hi there. I have a new one-page site in progress and it has one particular image that looks awful on mobile devices. The page is here and the image is the one halfway down the page of a man speaking on a stage.

It looks good on my computer, but on my phone it just shows a small section of the photo.

Can anyone tell me how I could make it look better on the phone? The theme allows me to add custom CSS in the options.

Thanks.