1) Create a TV call [*sectionImage*]. Default value @INHERIT. Type raw text.
2) Structure the top page of each section as a folder document.
3) In your template, place the following bit of code: <img src="[*sectionImage*]" alt="" />
4) In the top folder sections, on the sectionImage TVs, insert the image including path.
What it does:
@INHERIT is an "@binding" that looks up the document heirarchy until if finds a non-blank value in that branch, and applies that value to every document. You can override it on a per-page basis if you like. @INHERIT is one of MODx’s hidden gems.