There are 2 ways I can think of you can arrange this depending on your needs. They are:

1.) randomly select any of the albums photos

2.) have the content editor select which photo is displayed on the photo page

To achieve this:

1.) try something like this:

<msxsl:script implements-prefix='yourprefix' language='CSharp'>
    <![CDATA[
public int randomNode(int limit)
{
DateTime tmeNow = DateTime.Now;
int mls = tmeNow.Millisecond;
Random rndNumber = new Random(mls);
return rndNumber.Next(1, limit + 1);
}

]]>
  </msxsl:script>

  <xsl:template match="/">
    <xsl:variable name="AdGroup" select="/macro/source/node/@id"/>
    <xsl:variable name="collection" select="umbraco.library:GetMedia($AdGroup, 'true')/node"/>
    <xsl:variable name="selectedNode" select="yourprefix:randomNode(count($collection))"/>

    <xsl:for-each select="$collection">
      <xsl:if test="position() = number($selectedNode)">
        <img>
          <xsl:attribute name="src">
            <xsl:value-of select="./data[@alias='umbracoFile']"/>
          </xsl:attribute>
          <xsl:attribute name="alt">
            <xsl:value-of select="@nodeName"/>
          </xsl:attribute>
        </img>

        </xsl:if>
    </xsl:for-each>

  </xsl:template>

2.) Within the Settings section of Umbraco you can add node types in the Media types section the same as you would for document types. Create a new node type similar to Folder called Album. Add a media picker property to album and allow albums to have image nodetypes.

Cheers

Paul

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