My MdDlgHelp_xsl.xsl does not even include BackgroundImage as a variable. You can add it, though.
Change the line: <xsl:variable name="BackgroundColor">White</xsl:variable>
to: <xsl:variable name="BackgroundImage">url(H:\Docs\Thumbnails\Jasper Map_thumb.gif)</xsl:variable>
*change the above url to point to your image.
Now, find and change the following line: background-color: <xsl:value-of select="$BackgroundColor"/>
to: background-image: <xsl:value-of select="$BackgroundImage"/>
Finally, go into your tool properties and point the Stylesheet to your modified file.