Hi Andrew,

In the future you may get better responses if you post ASP.NET questions to
the microsoft.public.dotnet.framework.aspnet newsgroup.

If you're generating the image on-the-fly then you can cache the result
programmatically:

public Bitmap YourImage
{
   get
   {
       if (Cache["YourImageKey"] == null)
           Cache["YourImageKey"] = GenerateYourImage();

       return (Bitmap) Cache["YourImageKey"];
   }
}

"Caching Application Data"
http://msdn2.microsoft.com/en-us/library/6hbbsfk6.aspx

To allow IIS, proxy servers and the user agent (web browser) to cache the
request output you can use the following code, for example:

Response.Cache.SetCacheability(HttpCacheability.Public);

"How To: Set a Page's Cacheability Programmatically"
http://msdn2.microsoft.com/en-us/library/z852zf6b.aspx