Changing shading on a map

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ASP.NET

I've got a black and white map of office locations/desks. I'm pulling data from a database based on location/desk.

The map is one single image, and I'll be drawing hot spots for each location to link to the location specific data.

I need to update the locations (desks) on a map when it loads from the DB with shading based on the data I pull from the database. So instead of seeing an all white and black image, you'll see some areas with shading, but the black needs to show.

Am I looking for many transparent images on top of the main image? What should I research?

Thank you,

Patrick