Property ObjLayer[] specifies the object layer (where it is displayed and the base of its coordinates).
Assuming you have a layer 1 (the layer just over the background layer), you can create a line over it writing:
ImageEnVect1.ObjLayer[-1] := 1;
ImageEnVect1.AddNewObject(iekLINE, Rect(0,0,200,80), clBlack);
Index -1 means "index of next object that is created.