displaying images in in PDF's with fop

I'm using XSL stylesheets and XML documents to generate PDF files with
xsltproc, then fop.  The problem is, I can't get images to show up in the PDF
(the text showing up in the fo file, though).  I've tried the following:

<fo:external-graphic src="url(/path/to/image.jpg)" />     
<img src="http://path/to/image.jpg" content-height=".2in" content-width=".5in"
overflow="visible"/>
<fo:external-graphic src="file:///http://www.path.to.image.jpg"
content-height="70%" content-width="70%" overflow="visible"/>
<fo:external-graphic width="50pt" height="50pt" overflow="hidden"
src="image.jpg"/>

I also tried a number of other variations of <img src>, <fo:external-graphic>,
and even <mediaobject>.  I tried absolute and relative paths.  If I try to use
the path to anything other than something that resides on the server I'm
currently developing on, such as the google logo, I get a failed RPC error. 
Since I don't get the same error for files on my development server, I can
only assume that the file is being accessed - its just not showing up in my
PDF. 

I've seen a lot of posts from people about getting images to display in their
PDFs, but the solution was generally one of the ones I listed above.  Am I
missing something?

Thanks.