Icon (image) control front page

Hi
https://www.contentsale.ca

I’m not clear on how the images for the 3 links (sample pages) on the homepage are controlled or set up. Of course I want 3 different images but as of today I can only get the same image across the three links. I know the answer is simple, nuff said ;-))

Can someone direct to a doc which explains this.

Thanks…