inserting image in dynamic .prep file without saving image first

Dear list,

following this recipe ( http://stackoverflow.com/a/19160507 ), I am
trying to emulate the "not saving an image on the hard disk" approach
for z3c and preppy in an dynamic .prep file.

I fail and hope for your wisdom.

Two questions:

a) what object type does z3c.rml expect for images?
b) How would you suggest do I approach the goal of not saving images on
my hard drive yet inserting them in a pdf via z3c?

Thanks for your guidance and creativity!

David