You are allowed to attach someone else's photo provided the following are true:
-It is your own object. You can't attach someone else's photo to someone else's page.
-The image score is above 5%. You can't attch anything below that mark because it doesn't appear o search.
Since those are true for this, simply go to "Add Image" -> "Attach Existing" then search for the image and attach it. Notice that it doesn't say "searching only your images."