Often, my clients want a cover image on the presentation that says it all: the entire message of the presentation in just one smart visual. There are 2 problems with this approach:

• A technical one. The ideal image will probably not exist in some stock photo site, so there is significant photoshopping and editing required to get that elephant to balance on a skateboard while enjoying the benefits of flexible ROI. This image is unlikely to look good from a technical point of view.
• Even if you were to make this happen, it is highly unlikely that the audience who walks into the auditorium while sipping a coffee will actually understand what it means.

Lower the ambitions, and pick a professional looking cover image that is somewhat connected to what you are going to talk about and use your presentation to get the full message out.