You can easily insert image attachments/files that are related to a record into your PDF document. Paste the following code into the Source of the template editor where you want your pictures to appear. The LIKE test in the where clause might need to be adjusted.
Image Files
For image files, use:
<!--{{!
<lineitemsSOQL><class>none</class>
<soql>select ContentDocument.latestpublishedversionid from contentdocumentlink where linkedentityid='{{!ObjectId15}}' AND (ContentDocument.FileExtension LIKE '%jpeg')
</soql>
<column prefix="<img src='/sfc/servlet.shepherd/version/download/" postfix="' width='150px' />">ContentDocument.latestpublishedversionid</column>
</lineitemsSOQL>
}}-->
Image Attachments
For image attachments, use:
<!--{{!
<lineitemsSOQL><class>none</class>
<soql>SELECT Id FROM Attachment WHERE ParentId='{{!ObjectId15}}' AND (ContentType LIKE '%jpeg')</soql>
<column prefix="<img src='/servlet/servlet.FileDownload?file=" postfix="' width='150px' />">Id</column>
</lineitemsSOQL>
}}-->