Add watermark on image export

You will have to create an image style which applies your logo. Then you can use this style when exporting.

Step by step:

1. Preparation

1. Place your watermark file (as .png or .svg) in the folder ~/.config/darktable/watermarks (for Linux) or C:\Users\<username>\AppData\Local\darktable\watermarks (for Windows). If the folder doesn't exist, create it.
2. Restart Darktable

2. Add watermark to one picture

3. Go to the darkroom view for a picture
4. Select "watermark" ("Wasserzeichen") from "more modules" ("Weitere Module") (this is located at the bottom of the right panel)
5. Select the name of your watermark file for "marker" (it should be included in the dropdown)
6. Adjust position, size, opacity and so on
   (I prefer 70% opacity, 15% scale, bottom left, 0,015 y offset)

3. Create a style from the picture with the watermark

7. Once you're happy, go back to the lighttable view (your image should still be selected)
8. Open the "styles" menu in the right panel
9. Click "create..."
10. Make sure only the watermark module is selected, add a name and description and hit "save"

4. Use the new style for exporting

11. Now select a different picture
12. Open "export selected" in the right panel of the lighttable view
13. Select your newly created style for "style"
14. Export