How to add background image like this?

MikeC

Custom for each item.

Pollux

As [deleted] already pointed out, having custom backgrounds for each discussion will soon blow up the size and negatively impact the performance of your site.

However, if you just want to highlight a small number of discussions you can do it like that:

.DiscussionList-discussions li[data-id="24291"] {
    background: #ddd;
}

The data-id is the number of your discussion according to the URL and instead of #ddd you can easily apply an image as well.

[deleted]

Whilst this could work if you targeted the .DiscussionListItem class, defining a unique background for each is where you'd run into issues unless you specifically target a unique ID. In addition, taking this route is going to significantly impact page load times, so whilst it might look nice, it's not really practical from the usability standpoint.