Try it!
Adding a graphic as a watermark behind the text of your slides is a great way to display a logo or other meaningful image. PowerPoint lets you adjust the picture's transparency after you put it on the slide.
To add a logo to all the slides:
Select View > Slide Master. Scroll to the top of the thumbnail pane on the left, and select the first item, the slide master.
Select Insert > Shapes. Pick a shape and then select and drag to draw the text box on the slide master. It should be about the same size as your logo.
Note
If your logo is circular, use a circle shape.
Right-click on the shape and select Format Shape to open the menu.
Inside Format Background, under Fill > Picture or texture fill.
Select Insert under Picture source and insert your logo image. Adjust darkness of the logo with the Transparency slider.
To remove the border around the image, select the logo and then right-click > Outline > No Outline.
Exit Slide Master. All slides except title pages will have the logo.