Only make the image url absolute if it isn't already

resources/xslt/absolutizeImagePath.xsl

@@ -12,9 +12,23 @@
               doctype-system="http://www.daisy.org/z3986/2005/dtbook-2005-3.dtd" />
 
   <xsl:template match="dtb:img/@src">
-      <xsl:attribute name="src">
-	<xsl:value-of select="concat($absolute-image-path, '/', .)"/>
-      </xsl:attribute>
+    <xsl:attribute name="src">
+      <xsl:choose>
+	<!-- Only make the url absolute if it isn't already -->
+	<xsl:when test="matches(., '^([a-z+]+:)?//', 'i')">
+	  <!-- we are looking at an absolute url, no need to make it absolute -->
+	  <xsl:value-of select="."/>
+	</xsl:when>
+	<xsl:when test="starts-with(.,'/')">
+	  <!-- we are looking at a relative url starting with '/' -->
+	  <xsl:value-of select="concat($absolute-image-path, .)"/>
+	</xsl:when>
+	<xsl:otherwise>
+	  <!-- we are looking at a relative url -->
+	  <xsl:value-of select="concat($absolute-image-path, '/', .)"/>
+	</xsl:otherwise>
+      </xsl:choose>
+    </xsl:attribute>
   </xsl:template>
 
   <!-- Copy all other elements and attributes -->