How to add a watermark image
Step 1
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.
Step 2
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).
Step 3
All images uploaded from this point on will now have the watermark applied.
Feature is available as of 1.7.0