I do something a bit like it. I make the map, do the screenshot, and convert it into a file of the right size. Then I put it up in a directory where my buddy has made a simple CGI-script that makes a map out of an HTML table with regular-sized cells and picks the image I uploaded as the background. As long as I remember that 20 pixels = one five-foot square, I can have a big-honking outdoor map, with the HTML table serving as the grid.
And we're doing IRC, which is why it's CGI-script -- I can dynamically create and move people. For a play-by-post game, you can just do it in normal HTML and move people yourself as the players send you requests.