But, many presenters still make the mistake of inserting a picture as is, as a huge file right off the camera. If your presentation contains many pictures this can make for a monstrous file size and possibly crash the presentation during the show.
Your best practice is to optimize photos prior to inserting them into your presentation. But, the good news is that you can still reduce the overall file size of the presentation by compressing all the photos, after they are on the slides. Read more on compressing photos in PowerPoint.