I have a designer from afar sending me pdf's for print. We don't print pdf's directly, we import them into Corel because 9 times out of 10 they're not sized right or have prepress marks etc. Why do the photos come in like a puzzle? This is in wireframe btw.
Myron,
The rectangles are how the bitmap is stitched together. Likely those rectangles are clipping masks for the parts of the images they display. Not always, sometimes they are literally the images broken up into those rectangles. Either way, it's how the bitmap was created on-page at the time the PDF was made.
The outline of the image itself is a clipping mask. Typically because the image had transparency.
Mike
***Edit to add. In Acrobat, you can click on the image and it will usually highlight any rectangular stitched bitmap pieces-parts.