You must add a title placeholder to the slide, so use the "title only"
layout instead of the blank one. Then drag the title placeholder off the
edge of the slide so it doesn't actually show in the presentation when it's
in slide show view.
You could also create a specific slide master for this -- drag the title
placeholder off the edge of the slide in the master. You'd then apply that
slide design to all your graphic slides and then use Format | Slide Layout
to apply the "title only" layout.