Yes, but perhaps not in the way you would expect.
If you are creating an HTML output (HTML Help, WebHelp) you can set a
background image to the topic body. Find the image you want and assign
it to the BODY. You then have a watermark on every page that uses that
Hopefully Willam won't mind my offering a teensy bit of clarification here. Note what he said about pages using that "style sheet".
To amplify that a bit, I'll spell it out because it's a wee bit on the tricksy side to locate exactly how to make the edit.
Look in your Project Manager pod to find your Style Sheet.
Double-click your style sheet to open it up in RoboHelp's Style Sheet editor.
Expand to the Other category, then click to select Background+Text(BODY)
Now in the lower left corner, click the Format button and choose Borders and Shading.
Then click the Shading tab