Office 2010 has a great new feature that is especially useful in PowerPoint.
Click on the Remove Background button on the Picture Tools Format contextual tab. Drag the handles to include as much of the picture area you wish to keep. Then fine-tune the retained and deleted areas with the Mark Areas to Keep and Mark Areas to Remove buttons. Don’t try to outline the exact areas to adjust. Try clicking a diagonal line across those areas instead. It may take several tries to get it right. However, it’s worth the effort for the versatility it allows.