Here is my concern. I have the logo image along with the entire header section of the site in an 'included' file. How do I present that image in the image sitemap? Should I give the URL of the 'included' file that only contains part of each page and as such wouldn't make much sense to a user who would get it in a search result? Or is there another way of providing Google with the URL to that image?
You probably don't need to at all. Why are you asking? It might provide some more context.
No. Sitemaps are intended for indexing pages, not the individual bits that make them up.
If you happen to have a page on your site that specifically provides things like brand assets, then do it there and include it in the image tags for that page. Some companies do this for press use, for example.
If this is important to you, then I'd probably just list it under the home page. I would not recommend including it for every page, just because it technically happens to appear on all of them. That'd be a waste of space and possibly trigger some alarms(that's speculative).
I'm not aware of anything "official" or widely-supported at this time.
But as a bit of trivia, I'll mention