Well, there are a few issues here.

   First, ToDouble is a method, so you have to do this:

Image1.BackColor = System.Convert.ToDouble(Session["backcolor"]);

   Then, you have to make sure that there actually is a backcolor entry.
If the call to Session returns null, it will throw an exception when passed
to ToDouble.

   Finally, the BackColor property is expecting a Color structure, so you
should convert whatever value is in "backcolor" to that.