Solution 0
I was able to update the imagemapster author's example and obtain the desired result.
CSS
#mapster_wrap_0 {
margin: 0 auto;
}
Example: http://jsfiddle.net/jQG48/399/.
mapster_wrap_0
looks auto-generated so there might be a cleaner way to accomplish this centering.
Solution 1
Creating a wrapper div around your map appears to work as well.
<div id="MapWrapper" style="margin:auto; width:75%;">
<img src="https://github.com/jamietre/ImageMapster/raw/master/examples/images/usa_map_720.png"
usemap="#usa">
</div>
Example: http://jsfiddle.net/jQG48/400/.