How do i make a mobile-friendly theme show the right stuff?

Hi
I’m using Twenty Seventeen, which I know is mobile-friendly, but what I don’t know how to do is make the contents so. What I mean is how do I ensure that there’s available graphics that are dependent on how the page is viewed? By that, what I mean is how can I create the main page for what’s shown here to have the graphic shown that depends on how it’s looked at?
If you look at the page now, on a computer, it’s not too bad. However, if you flip to a phone, it’s completely non-sensical, because the graphic is far too large. Is there some way that I can put in multiple graphics, and what’s shown is called by what’s likely CSS code?

The page I need help with: [log in to see the link]