You can add a logo to your presentation in two ways.         

If you do not want your logo to appear on all slides, use the “Picture” button on the “Insert” tab to locate and insert the logo on selected slides.

For the logo to appear on all slides, insert it in the master slide. To do this:

1.       Go to “View” tab and click “Slide Master”

2.       Select the main slide master (the first slide in the slide master list)

3.       Go to the “Insert” tab, click “Picture”; locate the logo and insert

4.       Position your logo in the space you want it to appear

Download a PowerPoint template with great pictures here