Although PDF documents are usually a finished product that completely embeds fonts and images, Adobe Acrobat Professional has extensive tools that allow you to directly edit and add new texts or multimedia to PDF files. You can not only insert sounds, videos and flash files, but you can also easily add images to any part of your PDF. Here is a step-by-step guide.
Sharing this mostly so I have a convenient place to look this up the next time I need to do this. It is beyond comprehension why such a simple task is so stupidly and unnecessarily obtuse in Acrobat.
The worst part about this is that the default print settings in Reader don't include stamps, so if you send a PDF with images in it, the images won't print by default.
There's gotta be an easier way but knowing Acrobat, there probably isn't.