Hello,
You have to use some CSS to do this. 
On Common (CSS) Mobile and Desktop view add a 100% width to the image.
#custom-ad {
.custom-ad-link {
img {
width: 100%;
}
}
}
On Desktop (CSS) view add a maximum width to #custom-ad id to set the banner width same as topic avatar + topic body. Which is going to be the image 100% width, what we add above to the common.
#custom-ad {
max-width: calc(45px + 690px + (11px * 2));
}
45px = Topic avatar width
690px + (11px * 2) = Topic body width and the left 11px and right 11px padding used on .cooked class.
It will looks like 
Desktop view (full)
Desktop view (responsive)
Mobile view
