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 (i.e. signature image). Then, single-click on the pasted image — a boundary will appear around the overlapped image.
Now, Press Command+C to copy the image
Now paste it onto the PDF file.