Convert the image to PDF with ImageMagick:
convert image.png image.pdf
Place the image to the desired position in a PDF file (e.g. A4 sheet):
Note: this produces image-pdfjam.pdf.
pdfjam --paper a4paper --scale 0.9 --offset '0cm -5cm' image.pdf
Add image PDF to base PDF with pdftk:
pdftk base.pdf stamp image-pdfjam.pdf output out.pdf
Source: http://stackoverflow.com/questions/20531079/adding-an-image-to-a-pdf-with-pdftk