<ContentView>
   <Grid>
     <Image Source="{Binding MyImageSource}"/>
   </Grid>
</ContentView>

<ContentPage xmlns:local="namespace;assembly">
  <ContentPage.Content>
    <OnIdiom x:TypeArguments="View">
     <OnIdiom.Phone>
       <ListView>
          ... on list item tap do Navigation.PushAsync(new PhotoItemPage{BindingContext = itemTapped} );
       </ListView>
      </OnIdiom.Phone>
     <OnIdiom.Tablet>
       <Grid> // Add two columns
         <ListView Grid.Column="0">
            ... on list item tap do PhotoItem.BindingContext = itemTapped
         </ListView>
         <local:PhotoItem Grid.Column="1"/>
      </OnIdiom.Tablet>
    </OnIdiom>
  </ContentPage.Content>
</ContentPage>

<ContentPage xmlns:local="namespace;assembly">
   <local:PhotoItem>
</ContentPage>