It turns out just binding the BindingContext property of an inner ListView to the pictures List property of the listed object is all I need:
<ListView ItemsSource="{Binding .}" HasUnevenRows="True">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<StackLayout>
<Label Text="{Binding Prompt}" />
<ListView BindingContext="{Binding Pictures}" ItemsSource="{Binding .}">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<Image
HeightRequest="80"
Source="{Binding PictureName, Converter={StaticResource PictureNameToImageSourceConverter}}" />
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
</StackLayout>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>