This is sometimes caused when FlashPlayer doesn't think it has permissions to alter the bitmaps. It draws the bitmap data as white, as opposed to drawing the actual image. This appears to be a defect in FlashPlayer. This might also be a browser-specific problem. For example, many times the slideshow will look fine in Safari, but it will show the white images in FireFox or Chrome.
For now, turn off smoothing by setting "Options / Smoothing" to "Off", and re-export your slideshow.