Make sure that the font is installed on your server.
Also, your code will fail horribly if two people view the page at the same time.
You need to create a .ASHX handler that takes parameters in the query-string and dynamically serves the image.
You will run into memory troubles with your code. All GDI+ objects need to be carefully released or they show leaking behavior (even though the GC will eventually clean up via finalizers this can be too late, since the amount of unmanaged memory left sitting unused may cause the application to break earlier).
Also, you may want to use a special IHttpHandler which processed the requests for such "dynamic text" instead of creating "static" files.