What does "Initializer for 'source': unrecognized compile-time directive" mean

Hello I am currently making a splash page for an app I am making and when i put all the code to gether it says "Initializer for 'source': unrecognized compile-time directive" I am not sure what I am doing wrong.

Below is the code:

<fx:Declarations>

                    <s:SplashScreenImageSource

                                    source="@embed('assets/TheSolarSystemSplashPage.jpg')"/>

</fx:Declarations>

I am having trouble on the 3rd line.

Thanks for the help.