|It would help if we knew what version of PowerPoint you were asking about, but anyway...|
Draw an oval or a square or any shape over the picture and format it for No Fill.
That will leave just the "line".
Format the line as you like.
Another option is the drawing toolbar which will allow you to use the freehand "pencil" to draw any kind of shape you like. End where you start and the shape will fill in. Format it as as No Fill to leave just the line.
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