When converting a powerpoint presentation to PDF, using Windows Acrobat Professional 6.0, every image was given a 1 pixel black border. In the original powerpoint the images were a mixture of jpegs, word-generated art, etc. None were supposed to be bordered.

It turned out that the only way to get rid of the borders was to go back to the powerpoint, select each image, and change the "line color" to white instead of "No line" which is the default setting.