Why is collection view is not displaying images

The demo collection view code with an array of strings is displaying properly in my app but when l try the following code.

<CollectionView.ItemsSource>

            <Grid.RowDefinitions>
                <RowDefinition Height="Auto" />
                <RowDefinition Height="Auto" />
            </Grid.RowDefinitions>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="Auto" />
                <ColumnDefinition Width="Auto" />
            </Grid.ColumnDefinitions>
            <Image Grid.RowSpan="2"
                   Source="superman.jpg"
                   Aspect="AspectFill"
                   HeightRequest="60"
                   WidthRequest="60" />

</CollectionView.ItemsSource>

it doesn't work. Can you let me know the reasons why