You can customize the “Your message was sent successfully. Thanks.” message with-in the the form’s edit page at the bottom
so add something like this:
Your message was sent successfully. Thanks.</p>
<p><a href="http://pages.sbcglobal.net/bluealbino/SYP/images/mrburns-oh.gif"><img width="234" height="369" alt="" src="http://pages.sbcglobal.net/bluealbino/SYP/images/mrburns-oh.gif" class="alignnone"></a></p>
Update:
Just tried and it works with this exact code.