which is the best practice for placing graphics on a slide?
(a) use a large graphic to focus the audience's attentions.
(b) use graphics that reinforce the content.
(c) use two or more graphics per slide to increase interest.
All the choices depend on the slide and the last one is a must. the first one should be for the introduction and conclusion and the second one should be used to make ideas conveyed in the presentation easier to understand.
I believe that the answer depends on the nature of presentation; is it serious or funny, is it for a project, job or friends, etc. I would avoid option C in most cases (two pictures is ok, more is not ok).