Turns out PowerPoint may be trying to be more helpful than you want. I want to control when, and how many times, an image is compressed. The default setting for PPT 2007 is to compress images every time the file is saved. Here is my example slide, which has just one large image:

With the image selected (1) go to the FORMAT tab, then (2) click the COMPRESS PICTURES option

This brings up the basic dialog box, but we want to go into the OPTIONS

Finally, UNCHECK the automatically compress images option so you can control which images, if any need to be compressed by PowerPoint.

– Troy @ TLC