A friend solved this for me. Posting it in case it will help someone else.
The changes I made to
vba_cmps_adminfunctions.php (via ftp: /forum/includes/vba_cmps_adminfunctions.php) worked.
Additionally, navigation.php (via ftp: /forum/modules/navigation.php) needs to be edited in two places, as follows:
1. Find:
PHP Code:
$title = $navlinks['text'];
Replace with:
PHP Code:
$title = $navlinks['text'];
$image = $navlinks['image'];
2.
Find:
PHP Code:
$templater->register('title', $title);
Replace with:
PHP Code:
$templater->register('title', $title);
$templater->register('image', $image);
Regarding
adv_portal_navigationbits template, edit as follows:
Code:
<div><vb:if condition="$image"><img src="{vb:raw image}" alt="{vb:raw title}" align="top" vspace="1" /><vb:else />{vb:raw navmark}</vb:if> <vb:if condition="$link"><a href="{vb:raw link}"<vb:if condition="$native == 0"> target="_blank"</vb:if>>{vb:raw title}</a></vb:if></div>
Please note that mine is written so that links to anything outside of my site open in a new window/tab. Use align and vspace (vertical space) to adjust alignment of the images.
You should now have Image fields in your Admin CP when editing your Site Navigation module. Just input a relative path to your image. Example: /forum/images/custom/image.png
That's it!