Using one or two graphics on each slide provides a nice balance between text and images… but keep it minimal. Having too many will make the slide feel very busy, and could be more of a distraction than a complement to the text.
In addition, presentation programs like PowerPoint have a lot of animations. These can be used to transition between slides, or for each piece of content on a slide (the title, a text blurb, a piece of clipart, etc.). While they can add a nice touch, don’t overdo it! Having different animations for every slide transition and for each piece of content on the slide will take up a lot of time and be an eyesore.