Ok, the script is now updated to use MutationObserver.
Re: @jscher2000:True. I thought I already did in this script but apparently it I only added the MutationObserver in another, similar one. I'll update tomorrow :)
I'm sure there are others out there, but here is a script I created a while ago that should be able to do what you are looking for.It replaces any keyword on a page with different text or an image.
Re#comment-146f6f8cd17): @Marti:Marti is right. You don't really need a script for that. A simple custom stylesheet will be enough.Just an example to get you going: https://gist.github.com/LenAnderson/fa696934a59e8718c1ec