Try this:
1. insert a text box.
2. go to properties of the box and set the background color to none and the
line to none.
3. go to the top bar and insert a picture.
4. right click on the text frame, click on "order" and then "send to back".
5. now you can move the text frame around to the location under the text you
want over it.
note: you get a better effect if you edit the picture and either set it as
transparent or you adjust the contrast to make the image look like a water
mark.