[Theme: Pulse] Making a fixed background image responsive

Hi there. I posted this elsewhere, but I think this is the appropriate forum, after all.

I have a new one-page site in progress and it has one particular image that doesn’t resize 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, because it’s displaying the full-size image rather than resizing it for the smaller screen.

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.