You seem to be pushing Excel quite a long way outside its (or your) comfort
zone. Yes, you could certainly do all this in Excel, but really you'll need
much more than mild proficiency in VBA or you'll be struggling.

This is the sort of thing that browsers do all the time - simply use hyperlinks
as needed to navigate around the small and large maps. Even zooming would
probably be easier in a browser than in Excel. An additional benefit is that
you could host all the maps centrally and not need to distribute this to all the
machines (if they are on a common network or the Internet).

As for compression ... as far as I know, JPG files can't be compressed any
further. The only thing you could do to make the files smaller would be to
reduce either the resolution or the colour depth. Both of these would lose
information.

If you use a browser-based solution, you can use thumbnails for the small maps
and only send the full-sized versions of the ones the user wants to see.

Don't know if it helps, but when you hold down the CTRL key, the
scroll button on the mouse becomes a zoom slider...

I have an application in excel  which could be greatly enhanced by
having a map. How are yours placed/imported into the sheet?
The CTRL-scroll-wheel zoom works to three levels in this IE
browser when reading this group.
It zooms to many levels in my Excel 2002 and centers approx on the
cell selected. I pasted a jpg  into an Excel sheet and it zooms as
well.
Next try turning on the macro recorder and see what the CTRL-
Scroll-wheel yields.
Now if I can plot lines on the jpg, I have what I want.