Trigger element (XAML) is not supported in a UWP project

No, you don’t have Trigger support in UWP.

A workaround is to use DataTriggerBehavior with a ChangePropertyAction to accomplish the exact same thing.

  xmlns:Interactivity="using:Microsoft.Xaml.Interactivity"
  xmlns:Core="using:Microsoft.Xaml.Interactions.Core" 

<Button x:Name="MyButton" Width="140" Height="80" IsEnabled="False">
    <Image x:Name="MyImage" Source="Assets/xxx.jpg">
        <Interactivity:Interaction.Behaviors>
            <Core:DataTriggerBehavior Binding="{Binding IsEnabled, ElementName=MyButton}" Value="False">
                <Core:ChangePropertyAction TargetObject="{Binding ElementName=MyImage}" PropertyName="Opacity" Value="0.5" />
            </Core:DataTriggerBehavior>
        </Interactivity:Interaction.Behaviors>
    </Image>
</Button>

Note that you will need to include BehaviorsXamlSDKManaged from C:\Program Files (x86)\Microsoft SDKs\Windows\v8.1\ExtensionSDKs\BehaviorsXamlSDKManaged\12.0\. You will get a warning when you add the reference but just ignore it.

Update:
You should be getting an updated Behavior SDK from nuget now.

Option 2

You can always do the same thing in VisualStateManager. Open Blend and right click on your Button and select Edit Template, Edit a Copy and then specify the resource name you want and hit OK.

Then look for the Disabled VisualState and replace it with

<VisualState x:Name="Disabled">
    <VisualState.Setters>
        <Setter Target="RootGrid.(UIElement.Opacity)" Value="0.5" />
    </VisualState.Setters>
</VisualState>