Is there some kind soul who would tell me EXACTLY how I
can modify the configuration so that every occurrance of img in the source
being displayed is highlighted in red?
The help is very cursory and obtuse on this point for this non-programmer.
You could create a Syntax file for just this word. How to do this is well described in Help and there are several files already included that you can read, copy and modify for your own use. If you are already using a syntax file find some unused heading and put the word there.