HttpContext in the Designer

I've got a usercontrol in App_Code, and I load it onto my aspx page designer markup with the Register directive...problem is, the Render method makes use of HttpContext which is null and craps out leaving the control looking broken in the designer.  What I'd like to do is just render an image placeholder if the HttpContext fails...
 Now I have that working, but the path is static, I would like to make the path dynamic if possible...
(render method)

Image tempImage = new Image();
tempImage.ImageUrl=@"C:\Program Files\Telerik\Sitefinity3.5\website\Images\icons_temp.PNG";
tempImage.RenderControl(output);

...