How to change the front page picture

Hello,
This is the first time I am using WordPress and the first time I’ve ever tried to put together a wesbite myself. I have downloaded the Responsive theme, and so far so good.

I’m really struggling to understand how to change the picture from the cyberchimps picture to a picture of my choice.

I’m in the Customise > Homepage section and I can see the field where it says “Paste your shortcode, video or image source” – I tried to paste the image into that field but obviously that didn’t work. Would someone who understands the technical side of this be able to help a non techie please?

Many thanks,
Selda