PowerPoint does not have the rich image clipping and cropping tools that PhotoShop has. To take the background out of an image, you can set its background color to transparent and hope that the image edge come out reasonably clean.
Jose Arriaga recently started blogging about presentations on PowerPoint Symphony. He discusses an original alternative method: drawing a shape similar to an image and then fill it with the source picture as a background. Full details in his post here.
SlideMagic: a platform for magical presentations. Free student plan available.