You can add a border, such as a picture frame or a dotted line, around an image, shape, text box, or video, then modify the border’s thickness, color, and other attributes.
Note: To add or change a border for a chart, see Add a border and background to a chart.
Select an image, shape, text box, or video, then, in the Format 
sidebar, click the Style tab.
Click the disclosure triangle next to Border, then click the pop-up menu and choose a type of border (line or picture frame).
Use the controls that appear to adjust the look of the border.