You can merge publicly available images on your document:

  1. You can copy and paste any image directly into your template (including header and footer), that will be visible on every generated document.
  2.  You can also create a URL type field on master object to merge an image field. Use this field to enter URL of the image that you want to show on the document.
    1. You can also upload the image to Documents tab in Salesforce and copy and paste the link. Make sure you check the box Externally Available.
    2. You can paste the link to any externally available image, from your server, WordPress Media library etc. From your Document Action click “Edit Template Document” to edit the template.
      Image-Field
  3. From your Document Action click “Edit Template Document” to edit the template.
  4. Open object record and paste the copied image URL value in this field.
  5. From your Document Action click “Edit Template Document” to edit the template.
  6. Select the object and image field.
  7. Check the “Image” box then copy and paste Merge Field Code onto your template.
  8. Field will be displayed as an image on your newly generated Document.
    Image-Fiels-Merge