I think you might take a look at jpg2000. - There are some spectacular examples in the Library of Congress. "american memory" is the title of the site. 'General Maps' is the category. Extremely tedious to find & to navigate ,& you'll have to keep looking for & clicking on links till you get to the actual jpg2000 file download links. so, if i may -
This file format is supposed to enjoy widespread support these days. It seems to me that even though my OSX.3.9 is supposed to support it, i wound up using Lizardworks Expressview to open them ( i think Preview app choked on them.). This image reader is nominally for the MrSid (.sid) file format, but it opens jpg2000 as well.
Both files are very similar. They offer extraordinary zoomability. Some of the antique map facsimiles at Lib. of Congress are jpg2000 & some are .sid files . You can also find .sid files at the Beinecke library. (Check out the Voynich manuscript there, in .sid!).
You might not want to ask your surfers to install an oddball plug-in for a relatively rare file format like .sid, but if browser support for the more commonly supported jpg2000 is a problem, this viewer (Expressview from Lizardworks) opens them as a plug-in for web-browseres too.
They also make the software to create Mrsid files as well. As for software to make jpg2000, i believe it is supported on many image editors ( OSX.4 is supposed to include this function, so Windows must at some point as well.). Anyway, check these file formats out. I love playing w/ them, zooming in & out on maps that are 500 yrs old.