1. Activate Image Watermark
2. Make sure it’s updated to 1.1.1 release
3. Go to Watermark settings
4. Enable Manual watermarking
5. Go to Media Library.
6. On the left top part of the screen you’ll see Bulk Actions. Under that dropdown menu a new option should appear – Apply Watermark.
7. Select images (checkbox on the left of image title) you want to apply watermark to
8. Click Apply
9. Wait for the notice about how many images were watermarked
10. Done

If you don’t get any notice, check the images you select. If selected images are not jpg or png the watermark will not be applied.