Adding signature to images (for purposes of self attestation)

Many organizations in India require us to submit "self-attested" documents.  Self-attestation simply means affixing your signature to a photocopy of a document.  This same requirement has also been preserved when one uploads these documents online.  Here are the steps I followed in order to convert an already existing document image without self-attestation into one with a signature:

• Used https://www.scanwritr.com/app/ in order to create a pdf document  with a signature.  Note that the original file was a .jpg file
• On my Linux machine I ran the command pdftoppm foo_self_attested.pdf foo_self_attested -jpeg -singlefile to generate a .jpg file out of the pdf file
   

The resulting file was acceptable to the "National Voters' Services Portal" (I was trying to apply for my Election card)