I was a bit surprised when you said nothing works when you set AutoReverse="False" and RepeatBehavior="1x", so I tried it, and a single fade out works fine. Here is the xaml:
<Image.Triggers>
<EventTrigger RoutedEvent="Image.Loaded">
<BeginStoryboard>
<Storyboard>
<DoubleAnimation Storyboard.TargetName="imgSlot1"
Storyboard.TargetProperty="Opacity"
From="1.0" To="0.0" Duration="0:0:1"
AutoReverse="False" RepeatBehavior="1x"/>
</Storyboard>
</BeginStoryboard>
</EventTrigger>
</Image.Triggers>
However, I am not sure you want that to occur on the Image.Loaded event. And remember you can easily control that from C# or VB, even without a storyboard, similar to the following:
DoubleAnimation fadeoutAnimation = new DoubleAnimation();
fadeoutAnimation.Duration = TimeSpan.FromSeconds(1.0d);
fadeoutAnimation.From = 1.0d;
fadeoutAnimation.To = 0.0d;
imgSlot1.BeginAnimation(Image.OpacityProperty, fadeoutAnimation);
Hope this helps!