Why is the Image.Margin in a CollectionView template not creating visual space? - xaml

Question

We have a ListView bound to a List of objects of a type called Trial. Each Trial has a property, called Pictures, that is a List of objects of a type, Picture, that represents an image and some metadata. We are using a horizontal CollectionView within the ListView item template to display thumbnails of the images, if any. The problem is in getting a little space between the images. We've tried setting a Margin property value on the Image in the CollectionView item template, but the images are still appearing right next to each other as seen in the illustration.
This is the XAML describing the ListView:
<ListView x:Name="trialsListView"
ItemsSource="{Binding .}"
ItemTapped="Handle_ItemTapped"
CachingStrategy="RecycleElement"
HasUnevenRows="True">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<StackLayout Padding="20">
<Label
Text="{Binding Prompt}"
BackgroundColor="{Binding TrialType, Converter={StaticResource TrialTypeToColorConverter}}"
Padding="5" />
<Label Text="{Binding Response}" />
<CollectionView
BindingContext="{Binding Pictures}"
ItemsSource="{Binding .}"
ItemsLayout="HorizontalList"
HeightRequest="{Binding Count, Converter={StaticResource PicturesCountToHeightConverter}}">
<CollectionView.ItemTemplate>
<DataTemplate>
<Image
HeightRequest="80"
Source="{Binding PictureName, Converter={StaticResource PictureNameToImageSourceConverter}}"
Margin="10" />
</DataTemplate>
</CollectionView.ItemTemplate>
</CollectionView>
</StackLayout>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
And this is an example of output (from an iOS device):
I suppose we can enclose the Image in a Frame or some layout, but that feels like a 90s web hack. Surely there is a right way to get some spacing without resorting to adding screen elements? What do I not yet understand here?