Realized a bit more info might help... here is the master page:

<asp:Content ContentPlaceHolderID="cp_content" runat="server">

 HERE IS AN IMAGE : <umbraco:Macro PropertyAlias="HomePageImage" Alias="MediaOutput" runat="server"></umbraco:Macro>

</asp:Content>

...and here is quite a simple document type:

...and here is the xslt:

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

    <xsl:param name="currentPage"/>
    <xsl:variable name="propertyAlias" select="/macro/propertyAlias"/>
    <xsl:variable name="propertyValue" select="$currentPage/* [name()=$propertyAlias]"/>
    
    <xsl:template match="/">
        <!-- check to make sure the property value that we pass in is not empty -->
        <xsl:if test="$propertyValue!=''">
            <!-- localize the media nodeset into a shorter variable name for readability -->
            <xsl:variable name="mediaItem" select="umbraco.library:GetMedia($propertyValue, 'false')"/>
            <xsl:if test="$mediaItem!=''">
                <img>
                    <xsl:attribute name="src">
                        <xsl:value-of select="$mediaItem/umbracoFile" />
                    </xsl:attribute>
                    <xsl:attribute name="width">
                        <xsl:value-of select="$mediaItem/umbracoWidth" />
                    </xsl:attribute>
                    <xsl:attribute name="height">
                        <xsl:value-of select="$mediaItem/umbracoHeight" />
                    </xsl:attribute>
                    <xsl:attribute name="alt">
                        <xsl:value-of select="$mediaItem/@nodeName" />
                    </xsl:attribute>
                </img>
            </xsl:if>
        </xsl:if>
    </xsl:template>

What are we missing please?

Hi, there. The only thing I can think of is that you don't have your macro paramters setup correctly. Make sure you have a macro parameter with an Alias of propertyAlias set in your Paramaters tab found in the details of the macro that drives the above XSLT file. 

Your script looks valid otherwise...

HTH,
Nik

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