To add a watermark you will need to slightly alter your logo image.
Render your watermark image into a PNG file with the transparency adjusted around 15% - 50%. Once you have it, use the Image Tool to upload it. You can also adjust it to cover the areas that you like.
A PNG sample:
Here's how it will look like when you apply it to your plan:
To personlise your plan even more, you can also add a Title block to each plan.