Does this function not work if you do because mine doesn't work still do I need to add something to the code?
Firstly, is it enabled in your vBulletin Options? (Refer to Lynne's Post)
If it is then check your threadbit template. AdminCP > Styles & Templates > Style Manager > (From Right pane) Edit Templates > ThreadBit Templates > threadbit
Look for something like the following, it will be near the beginning somewhere, yours may differ slightly due to customisation:
<if condition="$show['threadicons']">
<td class="alt2"><if condition="$show['threadicon']"><img src="$thread[threadiconpath]" alt="$thread[threadicontitle]" border="0" /><else /> </if></td>
</if>
<td class="alt1" id="td_threadtitle_$thread[realthreadid]" title="$thread[preview]">
$thread[title_editable]
<div>
Make sure you have the bit in green, else edit it in if you don't have it.