Scanning - Bitmap Resolution - DPI
If you want to learn more about all this resolution/DPI stuff and how it applies to PowerPoint, start here:
Taj Simmons has an excellent tutorial on using bitmap graphics efficiently in PowerPoint in addition to the other tutorials on his already brimming-with-good-info site.
There's also KN Pepper's dpi tutorial.
For more general information about the subject, visit Wayne Fulton's ScanTips.com for a superb explanation of everything you need to know about scanning, dpi, resolution and the rest, for printing or screenshow/presentation purposes.
NOTE: The Units Conversion tool included in the FREE RnR PPTools Demo will help you convert measurements into screen pixels - very handy for calculating the optimum scan size for an image.