Fabio,
Rightclick on the object, from the menu choose save as picture, choose a
graphic format in the save as list e.g. jpeg or gif or png, give it a name
and save.
Now go to Format - Background - Fill - Image and choose your saved image,
hit Ok and apply to the entire presentation.
As an alternative, select your image on the slide, cut or copy. Go to View -
Master - slide master and paste your image there. Now with the image
selected, choose the Draw button on the drawing taskbar. Choose order and
send to back.
Now it should appear on each slide.
Hope this helps
Luc