I would suggest using UltimatePDF module for it. It’s a great module that will help you to add pictures and customise your documents in the way you want. It is definitely worth the price but if you don’t want to spend money and you are looking for a simple solution you can use a hidden option for it.
In your Dolibarr go to
in column name add
in column value add
and column comment is just for your reference so you know what this code is for.
More hidden options can be found here
