You can create your custom watermark backgrounds in PowerPoint using your own photos and pictures. This technique gives you the chance to design unlimited free powerpoint templates that you can use for your presentations with your own backgrounds as watermark.
In order to do this, we will first create a shape and fill it with a Picture. This method let us adjust the transparency of the shape and we can adjust the value to something like 80% transparent to make the watermark effect.
Using this same approach you can also embed your logo as a watermark background in powerpoint, which may be really very useful for corporate presentations. However, you should be aware that embedding pictures as watermark background may not be the ideal solution in some presentations, especially if you don’t pay attention to the contrast for the text and background colors.
In order to help explain this method, we have recorded this screencast explaining how to embed a picture in powerpoint as a PPT watermark background.
Another useful method to add watermark image to powerpoint 2010 was explained in an older post. You are welcome to review this other method and use the one that best fit into your needs.