1. Open the image using Preview.
2. Select the entire image by pressing Ctrl + A, copy it with Ctrl + C, and then paste it onto the same image.
3. You will now have a new object on the image.
4. Click on the newly created object and press Ctrl + C to copy the image object in Preview.
5. Open the PDF in a separate Preview window.
6. Paste the image object into the PDF.

PS: In step 3, ensure that you create a new image object; you cannot directly paste from the original image.

One more thing: If you want to send this PDF to someone, the image objects in the PDF can still be edited. A little trick to make them less easy to edit is to click the "Print" menu in Mac Preview. In the print window, there is a "Save as PDF" option at the bottom. Save a new PDF, and this new PDF cannot be edited in Mac Preview directly.