Mandeswori and Ujwal have found this difficult. Finally Mandeswori has found a solution and we have documented it here :
Use Mac Preview to view your PDF.
Have a signature image open as well.
Now Simply copying and pasting to the PDF may not work… So follow the steps below.
Press Command+C to copy the image.
Now, bear with us: This may seem strange, but it works. Press Command+V to paste the image, and the new copy of your image will cover the original image.
Press Command+C to re-copy the top image
Now paste on to the PDF file.