You could either place the picture on top of the slide, making it visible at the desired time (entrance effect) an then making it invisible again (exit effect).
Or create an individual slide just with the picture, which is not in the standard flow of slides but linked from the slide in question an then links back to the calling slide.
Edit: Linking at the end of a Slide seems not possible. This can be worked around by splitting the Slide at the point where the fullscreen image is to be shown, and placing the fullscreen image slide between them.
Scenario: You have a slide with 5 bullet points. The first slide contains only two of them, then you put the fullscreen image slide and behind that one a copy of the first one (but this one contains all five bullets), where the bullets one and two are already visble and the third one is the next to show up when you click.