As a photographer, you share your photos with your clients before they have paid for the service. You submit your work to your clients so they can choose their favorite images. In all these cases, there is a risk: that your clients take your images without your consent. The watermark is there for that: it places your logo or signature on each photo, in a way that is visible but not intrusive.
You create a watermark once, then apply it to as many galleries as you like. Your original files are never modified: the watermark is applied dynamically, at the moment the photo is displayed to the visitor. When a client downloads their photos (after purchase or upon delivery), they download the originals, without any watermark.
Go to Watermarks from the sidebar of your photographer space. Click the "Create a watermark" button and upload your image (your logo, your signature, some text — any image will do!) Preferably upload a PNG or SVG file (transparent background).
You can then configure three settings for your watermark:
Size: from 1% to 100% of the smallest dimension of the photo. A watermark at 50% will cover a good part of the image, while a watermark at 15% will be more discreet. It's up to you to find the right balance between protection and aesthetics.
Opacity: from 1% to 100%. At 100%, your watermark is fully opaque. At 30%, it is transparent and lets the photo show through beneath it. Many photographers use an opacity between 40% and 60% to stay visible without overwhelming the image.
Position: 9 possible placements on your image.
A live preview shows you the result while you adjust the settings.
By default, the watermark is applied to all “Selection” galleries.
You can also manually enable the Watermark option when creating or editing a gallery. You can select the watermark of your choice from a dropdown menu, for each gallery (if you have created several watermarks).
If you delete a watermark, it is automatically removed from all the galleries that were using it.

Avoid watermarks that cover 20% of the image
Avoid positioning your watermark over faces
Be careful with garish colors
Check its visibility on different backgrounds
Test it at different display sizes
Make sure it withstands cropping
Choose a legible font, even at very small sizes
Prefer the PNG format for your logo
Make sure there is enough contrast between your logo and the style of your images

Your originals are never altered
The watermark is applied on the fly, only at display time
You can create several watermarks and assign them to different galleries
The settings (size, opacity, position) can be adjusted at any time
Deleting a watermark removes it from all the galleries concerned