You can use the following objectUI code to display a large image in the dialog box:

<objectUI 

 name="ImagePreviewLarge" 

 useAttributesOrder="true" 

 iconName="ui_image" 

 iconSize="68">

 <default mode="Hidden" />

   <web 

                 maxRows="30" 

                 maxCharsPerLine="80" 

         />

         <tab name="titleTab" />

         <attribute name="preview">

     <ui 

                         mode="ReadOnly" 

                         labelPosition="None">

       <template 

                                 xmlns="http://www.teradp.com/schemas/GN4/1/XmlSchemaExt.xsd" 

                                 kind="Image">

         <objectUI>

            <web width="500" />

            <winclient>

              <Style xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation">

                <Setter Property="FrameworkElement.Width" Value="580" />

                <Setter Property="FrameworkElement.Height" Value="500" />

              </Style>

            </winclient>

          </objectUI>

        </template>

       </ui>

      </attribute>

      <attribute name="name">

        <ui mode="ReadOnly" labelPosition="Above" />

      </attribute>

      <attribute name="title">

        <ui mode="ReadOnly" labelPosition="Above">

          <template kind="SimpleString" useSpellChecker="no" xmlns="http://www.teradp.com/schemas/GN4/1/XmlSchemaExt.xsd" />

        </ui>

   </attribute>

</objectUI>

Should you want the image to resize with the dialog box, then use

<winclient>

  <Style xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation">

    <Setter Property="FrameworkElement.HorizontalAlignment" Value="Stretch" />

    <Setter Property="FrameworkElement.VerticalAlignment" Value="Stretch" />

    <Setter Property="FrameworkElement.Width" Value="Auto" />

    <Setter Property="FrameworkElement.Height" Value="Auto" />

  </Style>

</winclient>

Instead of:

<winclient>

  <Style xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation">

    <Setter Property="FrameworkElement.Width" Value="580" />

    <Setter Property="FrameworkElement.Height" Value="500" />

  </Style>

</winclient>