I am writing my autobiography which includes a number of photographs. I never know whether the pictures will show when I logon. I have re-added them a number of times.
It seems to be a bug which only rarely affects some people, and even then, seems to be very file specific. Try using linked images not embedded - that way at least re-opening the document will get back the images.
Search with images for more threads about it.
Old bug, see Impress images disappear randomly and spontaneously