I stumbled across a really useful post on Digital Inspiration addressing this issue here. To keep file sizes down I usually save images to disk before re-importing them in the presentation. Use PowerPoint to extract screenshots:
  • CTRL and PRT SCR to make a screenshot
  • Open a blank PPT page (or open any graphics software)
  • CTRL and V to paste the image in
  • Right-click and do "save as" in the desired format