You will need to create an xml with the base64 encoded string of the image in the xml and then merge the xml with xdp to generate the pdf with the image
Is your form based on the share point backed form data model?the data.json is the data from the submitted formyou can extract the individual elements from the data.json and insert the same into your SP list
https://experienceleague.adobe.com/en/docs/experience-manager-learn/cloud-service/forms/prefill-azure-storage/introduction This is for the cloud service, but will work for 6.5 as well