How to get the device screen size ?

Hello everybody ! I'm here because I've got a problem. I want to get the device screen height and width. I've tried

screenHeight = Ti.Platform.displayCaps.platformHeight;
screenWidth = Ti.Platform.displayCaps.platformWidth;

I work with a Samsung Galaxy S5, and when I do that, it returns me 1080*1920. But, when I try to move an image like this :

if(image.bottom < screenHeight)
//move the image

it doesn't work !

I'm just trying to make a Flappy Bird, and I can't do it because of this problem. My bird can always exit ofthe screen. More, I need to adapt my code for each type of Android phone (and IOS next). Can somebody help me ?

Thanks a lot.