Pictures with certain shadow settings may display incorrectly
Problem
If certain shadow settings are applied to a picture, the picture disappears and appears to be replaced by a solid-filled rectangle. The shadow settings that cause this are the two that create an embossed effect (the ones at the lower left of the Shadow Setttings tool on the Drawing toolbar).
For example, here's an image with one of the problem shadow settings:
Solution
The simplest solution, of course, is to choose a different shadow setting for the pictures.
Here's the same picture with the shadow setting changed to one that works:
Microsoft is aware of the problem and has made a registry fix available. You can read about it here:
Pictures that have certain shadow settings may be displayed incorrectly in PowerPoint 2003