You have that error because you are using two different namespaces.
For example:
Probably something like that:
System.Web.UI.WebControls.Image img = new System.Drawing.Image();
You should reference the same namespaces such as:
System.Drawing.Image img = new System.Drawing.Image();
-OR-
System.Web.UI.WebControls.Image img = new System.Web.UI.WebControls.Image();
Good luck,
OI