In Ubuntu-docs, both the DeskTop help and the Serverguide, the compiled html files, the css file actually, can end up looking for yelp-code.png. However the file is not present. Perhaps, more importantly, we don't want it anyhow. Because it isn't there, we end up with "file not found" log entries clogging up the server. Example (from my test server):

[Wed Oct 09 16:45:33 2013] [error] [client 192.168.111.101] File does not exist: /var/www/new/13.10/serverguide/yelp-code.png, referer: http://s15.smythies.com/new/13.10/serverguide/automatic-updates.html
[Wed Oct 09 16:45:34 2013] [error] [client 192.168.111.101] File does not exist: /var/www/new/13.10/serverguide/yelp-code.png, referer: http://s15.smythies.com/new/13.10/serverguide/configuration.html

The issue is in the file /usr/share/yelp-xsl/xslt/common/html.xsl
here:

div.code {
  background: url('</xsl:text>
    <xsl:value-of select="$icons.code"/><xsl:text>') no-repeat top </xsl:text>
    <xsl:value-of select="$right"/><xsl:text>;
  border: solid 1px </xsl:text>
    <xsl:value-of select="$color.gray_border"/><xsl:text>;
}

where icons.code maps to yelp-code.png from within the file /usr/share/yelp-xsl/xslt/common/icons.xsl

Now,
For whatever reason, yelp-build is supposed to copy yelp-code.png over to the build directory during compile. It does for the other yelp-bla.png files that are needed, based on the actual source files. The following extract is cut and pasted from:

https://wiki.gnome.org/Yelp/Tools

"When creating HTML, yelp-build also outputs CSS and JavaScript files, as well as PNG files for icons and watermarks. What it outputs depends on what's in your document. ... For CSS, yelp-build creates a separate CSS file for each top-level language. For icons and watermarks, which images are copied depends on what types of admonitions and other elements you use."

So, I think the root issue here is with the yelp-tools package (yelp-build).
However, I say again, we don't actually want the watermark file to be requested in the first place.