To give your picture a more polished look, add a border to frame it. Using this procedure, you can add a black or white border, 3D frame, one with faded edges, and other interesting styles to your picture.
Select the picture.
Under Picture Tools, on the Format tab, pick a style that best suits the picture.
Tip: Click the More button (as shown below) to see more picture styles.
Here’s some additional information about working with pictures in PowerPoint Online: