How to create a watermark
The best way to watermark your slides is to put the watermark directly on the images. If that's not possible, here's what you can do from Smart Slider 3's side.
Add a new Static Overlay
After you created the slider where you would like to have the watermark, add a new Static Overlay.
Static Overlay will be over all of your slides, so you don't have to add the watermark on all of the images.
Add the watermark as a layer
Add a new image layer select your image and move it to the position where you would like to have it. Save the slide.
That's it, if you preview your slider you'll see the watermark on all of the slides.