You can not select more than one layer at a time so you have to specify which layer to rotate.
This rotates all layers except for layer 0:
procedure TForm1.Button1Click(Sender: TObject);
iAmount := 90.0;
for i := 1 to ImageENView1.LayersCount - 1 do
You will have to try doing this in a timer youself because I do not understand what you are trying to accomplish with the timer rotation.