If you are a photographer who uses watermarks on your photos, you know that adding a watermark by hand on every image is a laborious and time consuming process. Instead of doing it one by one, it obviously is much faster add all your watermarks at once using a batch process. DigiKam probably has the easiest way of doing this in Linux.
Browse to the folder you want, then highlight all the images you want to add a watermark to.
Once you have those images highlighted, right click and select "Batch Queue Manager."
Under batch tools available, you have the option to select "Add Watermark."
Select "Add Watermark" and the Tool Settings dialog will give you options.
Here you can select the file you want to use as a watermark, the placement and the scale size. It really works quite well but may take you a try or two to get it just right.
DigiKam does a great job and really has some fantastic options for batch editing.