You need to add that as picture...
Choose the "View" menu and click on "Slide Master" to view the slide master, which is where you will make all the changes to affect all of the slides in the Presentation. If you wish to only add a watermark to select slides, just put your cursor on the slide where you wish to add the watermark.
Select the "Insert" menu, point to the "Picture" option and click on either "Clip Art" to create a watermark from clip art or "From File" to use an image that you already have created.
Choose the clip art or picture you want and follow the onscreen directions to insert the clip art into the master slide.
Right-click on the image you wish to change into a watermark and select "Format Picture" from the shortcut menu to bring up the "Format Picture..." dialog box.
Click the "Picture" tab, select the "Color" drop down menu and select "Washout" to make the image into a watermark. Click "OK" on the "Format Picture" dialog box to close the dialog box and change the image into a watermark.
Use your mouse to right-click on the image once again, point to "Order" and click "Send to Back" to send the watermark behind the rest of the content on the slides.
