This is an edit I have to my forum.
In vB 3.0.x it's a phrase edit rather than a template edit.
Search in phrases ==> for "sticky_thread_prefix" click the radio button next to "Phrase Text and Phrase Variable Name" case sensitive = no. Find.
when it comes up. click to edit. add the image source url in the English translation box in the following format: (it's just basic html)
Code:
<img src="your url here">
I am 99% sure that this would be the same in 3.5 but to be honest I don't remember. It's been a while since I played with the demo I had running on my test server.