ok I saw something ...
string MyURL = this.MainDataSource.CreateNavigator().SelectSingleNode("/my:myFields/my:Dropdown",this.NamespaceManager).Value;
if(MyURL!="")
this.MainDataSource.CreateNavigator().SelectSingleNode("/my:myFields/my:field240",this.NamespaceManager).InnerXml="<IMG xmlns=\"http://www.w3.org/1999/xhtml\" SRC=\"" + MyURL + "\" />";
You say me that the name of your dropdown field is my:Dropdown and I saw the name of your function is public void field239_Changed. There is something wrong.
can you Right clic on your drop down, programming, Changed event, and put the code in this function