As a photographer, I use Adobe Lightroom to apply my watermarks on photos during the last step of exporting my photographs (when I need to export multiple photographs)

Why do I use lightroom?

• Its already a part of my workflow
• I can define the position, transparency & size of the watermark
• The size is a percentage of the size of the photograph and hence word well with photos with different orientations and sizes

I still do use photoshop or other photo editing tools which support layers when I want absolute control of the size, transparency & position of my watermark.