The graphics I am saving are done using Powerpoint graphic objects (rectangles, circles, arrows, etc), so they are not bitmaps within Powerpoint. I'm saving them as PNG. This actually works very well except that I can't control the size of the resulting graphic.

In the publishing system where I need to use them, we publish both a PDF version and an online version. In the PDF version, the graphic is the size that I make it within Powerpoint (using the Ppt ruler at the top). But the online graphic is bigger by a factor of 220/96 or about 2.3. This is why I need Powerpoint to create the image at 96 DPI.