My problem is actually not a defect. However, the solution I describe here might help others and/or could ameliorate this plugin.
As to the other empty PDF problems: I experienced all the failures as you did, too.
By defining a "pathtocover" (in the main trac.ini, not an inherited one) variable and rewriting the path to the pic (as described further down) the W2PDF plugin did its job. But here is my question.
After laughing cordially you want to create a PDF from the wiki page which contains this picture. When you are me, that does not work.
Then, after trying to get it to work, you find out, that this:
actually works. You can create a PDF with pictures in it.
Now my question: is it possible to change this plugin so it finds relative links? This would save us a good amount of time since we have a lot of wiki sites that need to be rendered to PDF... hopefully without the need to change the image macros.