The documentation says in section about the img library that "By img.write(a) a pdf object number is allocated, and a whatsit node of subtype pdf_refximage is generated and put into the output list."
Imho this should be a "rule node of subtype image" 10.1.7 node mentions also a "whatsit node of subtype pdf_refximage" -- Ulrike Fischer http://www.troubleshooting-tex.de/
