Add images to existing PDF with Preview
Do as follows:

Open the image you want to paste in Preview.app
Select All (Command-A)
Copy (Command-C)
Paste (Command-V)

Now you have a copy of your image pasted above your old image. This is apparently meaningless, but the new copy is not just an image, but an object.

Click on the new image (round blue corners appear, no marching ants)
Copy (Command-C)
Paste on your PDF document. The image is an object, moveable and resizable. The original PDF is still a PDF, editable and all.