<umbraco:Item field="image" useIfEmpty="image" textIfEmpty="/css/images/dog.jpg" runat="server" />

<div class="imgbox">
                        <xsl:variable name="image" select="$post/image"/>


                        <img src="/css/images/dog.jpg" alt="" />
</div>

<img src="{$post/image}" alt="" />

<xsl:variable name="media" select="umbraco.library:GetMedia($post/image, false)" />
<img src="{$media/umbracoFile}" alt="" />