<xsl:variable name="pic1" select="substring-after(@publishingImage,'src=')"></xsl:variable>
<xsl:variable name="pic2" select="substring($pic1,1,string-length(substring-before($pic1,'style'))-1)"></xsl:variable>

<xsl:choose>
       <xsl:when test="not($pic2)">
          <!-- If src attribute does not have value-->
       </xsl:when>
       <xsl:otherwise>
          <!-- If src attribute have value-->
       </xsl:otherwise>
</xsl:choose>