PS CS Technique Merge 2 Maps

Question

I have 2 or more World Maps (tiff images) whose backgrounds are essentially the same as I add more maps to the pile.
Each map shows who's visited my artsite, and I'd like to merge the collection of maps to show all visitors. They change from month to month, as my map marker only saves the most recent 100 visitors.
I've tried various things with varying success ... for recent example, link to my artsite and look down on the main page for Recent Visitor Map.
Anyone have a technique that works well? nails it?
Need to line up the maps, some being bigger than others, depending on how I saved them, cut and crop. In the future, this could be standardized.
Hints welcomed.

Answer 1

A quick search using the terms
free website visitor maps
found this:
http://www.niftymaps.com/
From the FAQ:
Are there any custom map designs?
Those that have more than 20,000 friends or 100,000 markers on their map will be able to have a special custom map designed. Just let us know and we will get right to work. You will not have to worry about losing any of your current markets either because they will all be transferred to the custom design.