Easy! You can simply go to your studio set-up panel just as if you were going to upload a watermark before you published your gallery. Upload your watermark as normal, adjust the size and position to your liking, and PhotoDay will automatically add that watermark to the images you have already uploaded.

Just as an aside, if you did not originally upload a watermark, PhotoDay automatically adds one as a default, which can be replaced at any time should you wish to upload one of your own! Alternatively, if you decide you like our default watermark better than your own, you can switch back to ours by hitting the "Default" button right below the watermark upload button.