Facing ContentPage.BackgroundImage problem

I have develop apps for iOS using Xamarin about 3 months, and my APP almost completed.
Tested in iPhone6 iOS v8.1.2, and iPad2 iOS v8.1

Now facing a problem that my APP can't run in 2 of my devices (iPad mini iOS v7.1.2, and iPhone4 iOS v7.1.2)
My APP will become blank page, I found that the following line of code cause the problem...

using Xamarin.Forms;

public class page_Testing: ContentPage
{

    public page_Testing () {

            BackgroundImage = myImagePath; // this line cause the entire page blank, commented this line will run normally. 
            // other coding here ...

    }

}

Anybody facing the same problem before? Please help...

Thanks!