Upon the revival of this thread, I realized that I never did resolve this issue either.  I got pulled on to other projects.  All I ended up doing to keep it from happening was to disable the image editor.  

However, I went back to one of my test projects that still had it enabled and managed to recreate the error.  Only once though.  All subsequent attempts to use the image editor succeded, even after forcing a rebuild of my project(s) and restarting the webserver, etc.  

Because I was running it in debug mode, I managed to locate the exception.  The NullReferenceException that I mentioned in my orginal post occured in the Page_Load event handler of imageeditor_showimage.aspx on the line containing the call to the DrawShow constructor.

I hope some of this info helps.