Hi,
It is because you are missing a reference to yor gif in your manifest.xsf.
If you have a gif called: aa.gif.
In the xsl, you should have (as you did): body style="background-image:url('aa.gif')"
Then, in your manifest, you should add:
<xsf:file name="aa.gif">
<xsf:fileProperties>
<xsf:property name="FileType" type="string" value="resource"></xsf:property>
</xsf:fileProperties>
</xsf:file>
and you need to copy your gif locally....
Hope that it helps.
Regards,
Franck Dauché