How to add a watermark image
Upload a transparent PNG image to your Media Library. If you want it to have some opacity (transparency), make sure you save your PNG with the transparency already applied.
Go to Sunshine > Settings > Galleries > Watermark.
Select your watermark image from the drop down and choose the position you and the margin (how far away from the edge of the photo).
All images uploaded from this point on will now have the watermark applied.
Feature is available as of 1.7.0