I have this one Image and I want it to be on top of another image.
(window form application, c#)
In the code you have a few of calls you can make. BringToFront and SendToBack are probably the simplest:
yourControl.BringToFront();
yourControl.SendToBack();
Also you could control that from the control's parent container using Control.ControlCollection.SetChildIndex. For example:
// Bring control to front
MyForm.Controls.SetChildIndex(SomeControl, 0);
Otherwise just click "Bring to Front" or "Send to back" in the designer, as @KeithS has already answered.