I eventually figured out, that the part I want to edit is found as an actual XSLT file. Something I also have never worked with before...

I have researched this though, and the code looks correct, but some how the it's not retrieving the images from the media section. Can anyone shed light on this?

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE xsl:stylesheet [        <!ENTITY nbsp "&#x00A0;">]>

<xsl:stylesheet        
  version="1.0"        
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"        
  xmlns:msxml="urn:schemas-microsoft-com:xslt"        
  xmlns:umbraco.library="urn:umbraco.library"        
  xmlns:Exslt.ExsltCommon="urn:Exslt.ExsltCommon"     
  xmlns:Exslt.ExsltDatesAndTimes="urn:Exslt.ExsltDatesAndTimes"        
  xmlns:Exslt.ExsltMath="urn:Exslt.ExsltMath"        
  xmlns:Exslt.ExsltRegularExpressions="urn:Exslt.ExsltRegularExpressions"        
  xmlns:Exslt.ExsltStrings="urn:Exslt.ExsltStrings"        
  xmlns:Exslt.ExsltSets="urn:Exslt.ExsltSets"        
  xmlns:ferinet="urn:ferinet"        
  xmlns:random="urn:random"        
  exclude-result-prefixes="msxml umbraco.library Exslt.ExsltCommon Exslt.ExsltDatesAndTimes Exslt.ExsltMath Exslt.ExsltRegularExpressions Exslt.ExsltStrings Exslt.ExsltSets ferinet">

<xsl:output method="xml" omit-xml-declaration="yes"/>
<xsl:param name="currentPage"/>

<xsl:template match="/">   

<xsl:variable name="images" select="$currentPage/imageFolder"/>
<xsl:variable name="imgWidth" select="704"/>
<xsl:variable name="imgHeight" select="297"/>

<xsl:if test="$images">
  <xsl:variable name="mediaNode" select="umbraco.library:GetMedia($images, 'true')" />
  <xsl:variable name="numImages" select="count($mediaNode/RotatorImage [string(umbracoNaviHide) != '1'])"/>

  <xsl:choose>

    <xsl:when test="$numImages = 0">
    </xsl:when>
    <xsl:when test="$numImages = 1">
      <xsl:for-each select="umbraco.library:GetMedia($images, 'true')/RotatorImage [string(umbracoNaviHide) != '1']">

        <xsl:variable name="url">
          <xsl:choose>
            <xsl:when test="./externalLink != ''" >
              <xsl:value-of select="./externalLink"/>
            </xsl:when>
            <xsl:otherwise>
              <xsl:value-of select="umbraco.library:NiceUrl(./link)"/>
            </xsl:otherwise>
          </xsl:choose>
        </xsl:variable>

        <xsl:variable name="image" select="./umbracoFile"/>
        <a href="{$url}">        
          <img src="/ImageGen.ashx?image={$image}&amp;width={$imgWidth}&amp;height={$imgHeight}" height="{$imgHeight}" width="{$imgWidth}" border="0"/>
        </a>
      </xsl:for-each>   
    </xsl:when>

    <xsl:otherwise>

      <!-- SHOW SLIDES -->
      <div id="container">
       <div id="slideshow">
          <ul id="slides">
          <xsl:for-each select="umbraco.library:GetMedia($images, 'true')/RotatorImage [string(umbracoNaviHide) != '1']">

          <xsl:variable name="url">
            <xsl:choose>
              <xsl:when test="./externalLink != ''" >
                <xsl:value-of select="./externalLink"/>
              </xsl:when>
              <xsl:otherwise>
                <xsl:value-of select="umbraco.library:NiceUrl(./link)"/>
              </xsl:otherwise>
            </xsl:choose>
          </xsl:variable>

            <xsl:variable name="image" select="./umbracoFile"/>
            <li><a href="{$url}"> 
          <img border="0" style="" width="704" height="297">
          <xsl:attribute name="src">
        <xsl:text>/ImageGen.ashx?image=</xsl:text>
               <xsl:value-of select="$image"/>
        <xsl:text>&amp;height=297</xsl:text>
        <xsl:text>&amp;width=704</xsl:text>
          </xsl:attribute>
          </img>
        </a></li>
          </xsl:for-each>   
          </ul>  
   <ul id="nav">
           <li id="prev"><a href="#">&nbsp;</a></li>
           <li id="next"><a href="#">&nbsp;</a></li>
          </ul>      
        </div>      
      </div>
    </xsl:otherwise>
  </xsl:choose>
</xsl:if>


    </xsl:template>
</xsl:stylesheet>