Trying to set a fallback image on a product.
Trying to get it from parent (that is category)

[InitializableModule]
[ModuleDependency(typeof(EPiServer.Web.InitializationModule))]
public class ContentEvents : IInitializableModule
{
public void Initialize(InitializationEngine context)
{
var events = context.Locate.ContentEvents();
events.PublishingContent += EventsPublishingContent;

}

private void EventsPublishingContent(object sender, ContentEventArgs e)
{
if (e.Content is RouteProductContent)
{
var content = e.Content as ProductContent;
if (content.IconImage != null) return;

var parentContentReference = content.ParentLink;
var repository = ServiceLocator.Current.GetInstance();
var category = repository.Get(parentContentReference);
if (category.IconImage != null)
{
var clone = content.CreateWritableClone() as RouteProductContent;
clone.Property["IconImage"] = new PropertyContentReference(category.IconImage);

// have also tried with
clone.Property["IconImage"].value = category.IconImage;
// and
clone.IconImage = category.IconImage;

}
}
}

public void Uninitialize(InitializationEngine context)
{
var contentEvents = ServiceLocator.Current.GetInstance();
contentEvents.PublishingContent -= EventsPublishingContent;
}
}