First, find the template you are interested in - vboptions > General Settings > Add Template Name in HTML Comments > set to Yes . Then go back to your page and view the source code and you will see the name of the template called around your part of the code. Sometimes the template is the one mentioned at the very top of the page source.
Then, go to edit that template and do something like this -
Find the thread views:
HTML Code:
<td class="alt1">$forum[threadcount]</td>
And do something like:
HTML Code:
<td class="alt1"><if condition="$forum[threadcount] <= '50'">egg image code here<else />dragon image code here</if></td>