Relative path for images in FOP

I have this:

<fo:block>
<fo:external-graphic src="url('logo.png')" height="2.00in" width="5.00in" />
<xsl:apply-templates select="head" />
......

I am getting this in the logs:

2013-03-10 01:12:29,952 btpool0-1 ERROR apps.FOUserAgent - Image not
found. URI: logo.png. (No context info available)

And of course the image doesn't show on the generated pdf page. I
tried all possible combinations of the image path:

src="logo.png"
src="url('COB-INF/xslts/fo/logo.png')"

The only thing that worked was the absolute path. However, this is not
an option when deploying to the server. So my question is, how do I
get fo2pdf serializer to resolve relative paths ??

Thank you.