PDFResource and converting a resource that uses an encoded string as a png source

In brief - I have a snippet that receives a png image from JQplot as a string. I can display the image correctly in a standard page by setting the header as a png type, however, when I set the resource holding the snippet to create a pdf on the fly using PDFResource it breaks the conversion and just shows as a missing image. If I look at the source for the image it shows as the url of the current page.

If I modify the snippet to use a physical copy of the image instead of the string then PDFResource converts it perfectly.

I'm guessing I need to manipulate the headers somehow to allow PDFResource to convert it but my knowledge and experience is limited (imaginative and enthusiastic amateur) so I have no idea how to do that. I've been looking around but getting nowhere fast so any suggestions or advice would be greatly appreciated.

:-)