I'm sure that there's a best practice for this that I just don't know about, but what's the easiest way to create and apply a watermark to a large group of photos? (Preferably in GIMP or another free photo editing program)
Can you make actions in Gimp the way you can in Ps? If so you can just run an action to handle it. Me personally, it's part of my export preset in Lr so I don't even think about it.