The slide master feature in PowerPoint allows you to make global changes to many or all of your slides without having to make these changes to individual slides.
Below are several articles that will aid you to making the best use of your time by using the slide master in PowerPoint.
1. Slide Masters in PowerPoint
3. Add Pictures to the PowerPoint Slide Master
If you need to have the same photo/clipart (such as a company logo) on every slide, the most efficient way is to insert the photo onto the slide master. Each time a new slide is added to the presentation, the photo will be present.
4. Create a Picture Placeholder on the PowerPoint Slide Master
Questions about picture problems are common in my inbox. In the case of adding many photos, in the same location on each slide, you may wish to avoid doing this task individually. Using a picture placeholder, makes sure that all photos appear in the same spot on every slide.
Edit the slide number field on the PowerPoint slide master to increase the slide number size on all of your slides at one time.