How to watermark a photo
Upload an image and the Watermark tab opens with a sample copyright line in the bottom-right corner. Type your own text, up to 80 characters. Use the Opacity slider (10 to 100 percent) to decide how strongly it shows, and the Size slider to scale it relative to the image width. Pick any of nine positions, from the corners to the dead centre.
The watermark is white with a soft shadow so it stays visible on both light and dark areas. It is text only: there is no option to upload a logo image. If you want a coloured caption at full strength instead, use the Text tab, and you can have both at once.
Where to place a watermark
A small mark in a corner at 40 to 60 percent opacity is unobtrusive and works well for portfolios and social posts. It identifies you without distracting from the picture, but a corner mark is also the easiest to crop away.
For proofs you send to clients before payment, or images that are often reposted without credit, a larger mark in the centre at lower opacity is much harder to remove. It does get in the way of the image, so it is best kept for previews rather than final deliveries.
No visible watermark makes copying impossible. It is a deterrent and a credit line, not a lock. Keep your original full-resolution files private and share only the marked copies.
What to write in a watermark
Short works best: a name, a handle or a domain. The copyright symbol followed by your name and year is a common format. Keep the same wording, size and position across a set of images so they look consistent. All edits are rendered on the server at full resolution; PNG files stay PNG and others are saved as JPG.