Use this code:
<div class="cfs">

<div class="mboard">
<div class="title">
<h1><!-- |img_new_post| --> <!-- |name| --></h1>
<div class="descimage" id="forum<!--|forum_id|-->"></div>
<div class="topics">
<span><b><!-- |topics| --></b> topics</span>
<span><b><!-- |replies| --></b> replies</span>
</div>
</div>
<div class="mdesc">
<h2><!-- |description| --> </h2>
<div class="last">
<b>Last post:</b> <!-- |last_topic| --><br>
<b>by:</b> <!-- |last_poster| --><br>
<b>on:</b> <!-- |last_post| --></div>
</div>
</div>
<div class="subf">
<!-- |subforums_list| -->
</div>
</div>

In the css, add these and repeat for each forum ID:
#forumNUM { background-image: url(IMGURLHERE); }

Replace 'NUM' with a number (e.g. #forum1) that corresponds to the forum of the image you want. So for example, Forum ID #13 has its own bg url so you'll set it as #forum13 and paste in the url for it. Copy and paste that line for every forum. If you want a default background image, then just throw in a background-image style for .descimage.

You can check the unique ID number of your forum by accessing the direct URL for the forum, and grabbing the numbers at the end of the link.

Apr 18, 2021 5:00:53 GMT Desmond Miles said:

ninelie thank you so much for replying!

But hmm, it didn't seem to change anything? Is it possible there's something else in the stylesheet that may be overriding the image and preventing it from appearing? I know you wouldn't know from my screenshots alone, haha, but I'm just curious if there's something I can look for. 

Without the actual forum page for me to Chrome Inspect, there's nothing I can do because there is no full context on what I'm working with.

You can always PM me the URL to me if you don't feel comfortable sharing it publically. I can take a deeper dive this way and reach the core of the problem.