If you use the command-line weathermap instead of the Cacti plugin, then you can specify where the image and HTML files are generated to. You can then edit that HTML file, and change your command line to only update the image. However, you need to regenerate the HTML if you add anything to the map, so that the imagemap (mouseover) stuff works, because that's in the HTML file. This is what most people who do what you saw are doing. Quite often those maps don't have mouseover graphs, and this is why.
When it is integrated into Cacti, it always generates a partial HTML page in the output/ directory, and includes that when you go to the weathermap-cacti-plugin.php page, depending on the stuff after the question mark in the URL. It regenerates it every poller cycle though, so you can't really edit it.
Things like the fullscreen view mode assume that the image is the only thing on the page too, so whatever it is would not show there. Generally, adding text in the image is less hassle (unless it's a lot of text).
Weathermap 0.98 is out! & QuickTree 0.2. Superlinks is over there now.
Some Other Cacti tweaks, including strip-graphs, icons and snmp/netflow stuff.
(Let me know if you have UK DevOps or Network Ops opportunities, too!)