Sends the image to an HTTP stream.
SaveHttp can only be called in an ASP.NET environment as it internally
uses ASP.NET's Response.BinaryWrite method.
DispHeader specifies the Content-Disposition header to be included in the HTTP stream.
This header carries the file name information to be presented to the user when the
file is saved to disk on the client machine. This argument has to be in one of the following two formats:
"filename=somename.png"
or
"attachment; filename=somename.png"
The latter prompts the user to save the file to disk instead of opening it in-place in the browser.
Format is an instance of the .NET System.Drawing.Imaging.Format class and specifies
the format the image is to be saved in.
If this parameter is not specified, the image is saved in PNG format.