Please look at the PDF (http://zopyx.com/tmp/index.pdf).

The image (100% width) would fit on the bottom of the first page if it could be scaled down
automatically in some way...is there same magic in PrinceXML in order to move the image on the first page instead of specifying a smaller width in my CSS?

No, this isn't possible yet. Imagine if there was only a very tiny space at the bottom of the first page, then the image would be scaled down to microscopic size to fit. Maybe in the future there could be a mechanism to say "make the image small enough to fit, but not too small".

Try floating the image to the bottom of the page.
Floats allow the text to continue flowing.
Don't know if you can allow the picture to be in a different location though.

Jim Albright
Wycliffe Bible Translators