Eger,
Not in PowerPoint no, but you can work around it.
Crop your picture and save it as a jpg/png. Use the drawing tools to insert
an oval/circle. Use the fill effect to fill your oval/circle with the
picture.
Personally I use SnagIt which does let you select a part of your picture in
a circle/ellips shape. Save it in SnagIt as a jpg/png and insert it into
PowerPoint.