C# .Net and serving up a dynamic image.

I have an aspx page that I've written right here: http://www.51stmercs.com/sigimage/sig-gen.aspx As you can see, the page generates a sig image with some dynamic text written onto the image. My issue is that calling the page directly renders the image just fine, but I need to use the link in an IMG tag on a phpBB bulletin board. Is there anything special I need to do with my page to get this to work?