A more useful rule of thumb may be no more than 1 idea per slide. A good slide guides the viewer towards the essence of an idea, rather than listing of the idea's attributes. Try not to clutter a slide with too much text, graphics, or color. Don't put anything you are going to say on a slide - the audience is likely to be bored. It is a good idea to sometimes emphasize your points with the use of a graphic object.
Pictures can provide the appropriate context for an idea.
They are more visually stimulating and more easily remembered. Don't use clip art for clip art's sake - try to tie the picture into the idea you are presenting. Pictures should add to the idea being presented rather than distracting. If you do decide to use clip art, try to venture outside the clip art gallery. Some background and foreground color combinations are difficult to read.