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
