Preview add picture to pdf

You can't. It's not a PDF editor.

You can mark it up with annotations "on top of" the document, but not insert pictures into the actual PDF. The annotations do not include an option for putting any image on top of the document.

What Barney said. If for instance, I download a free trial of PDFpenPro 12 from their website, I can drag and drop an image from my Desktop onto any PDF that I happen to open. That's the difference between Apple's Preview, and a true PDF editor.