external-graphic src syntax

[Michael Engelhart]

Can someone show me the syntax to using
<fo:external-graphic>??
I have an image in the same directory as the stylsheet and have tried 
this syntax:

<fo:block text-align="center">
    <fo:external-graphic src="logo.jpg"/>
</fo:block>
this syntax:
<fo:block text-align="center">
    <fo:external-graphic src="file:./logo.jpg"/>
</fo:block>
this syntax:
<fo:block text-align="center">
    <fo:external-graphic src="file://./logo.jpg"/>
</fo:block>
and none of them work.  I always get this error:
[ERROR]: Error while creating area : Error with image URL: ./logo.jpg 
(No such file or directory) and no base directory is specified

I have compared this to the images.fo example document as well as the 
W3C XSL spec and it doesn't work.
I can sort of get it to work using an external url like 
"http://somewhere.com/logo.jpg"; but when I do it that way my application 
hangs and never comes back.

any ideas??