All you would do is type: I have XYZ set for sale. You can see what this set looks like here: (link)
Where I have the word link, you would paste the URL for the image in our gallery.
To do that, click on the set you want to show. In your browser's address bar, highlight the entire web address of that page you're on with the set.
Copy that address with either ctrl+c or by right clicking and choosing copy.
Go back over to your listing and where I have (link), paste the link you just copied with ctrl+v or by right clicking and choosing paste.