image:the sourceSize of image should be reset while changing the source of image

the sourceSize.width and sourceSize.height of image should be reset while changing the source and sourceSize of image,
the property sourceSize.width and sourceSize.height will change only when the sourceSize.width and sourceSize.height are not set.
so that i can not get the actual size of image when i change the source and sourceSize.width,height of image.
On the other hand ,in QtSDK_1.1,when i just set sourceSize.width and sourceSize.height,don't set width and height,
like this :Image {
id:picture //actual size 4000*3000
sourceSize:1920
sourceSize:1080
Component.onCompleted:

}
I found that the picture.width and picture.height are 1920 and 1080,not 4000*3000.