I have seen this reported on ticket #33164, but thought (since this is a
work-stoppage for me) I would add some more information and re-open this hot 
item. I cannot upgrade to anything past v20.1a because, after that point, the
PCL render output fails when you specify external-graphic in the xsl with a
ArrayIndexOutOfBounds error. This worked perfectly with all versions upto 20.1a 
and before after which point the above error occurs. I have tried every 
distribution version of fop since to track down the exact point this problem 
started and what I state is it.

This is not a problem with a limited renderring engine as the response to 
#33164 states as it did work fine before with the same monochrome jpg images.