Help! How to select a part of the picture with Qml?

[itviewer]

just like css :

@background:url(images/123.gif) 50px 74px;@

now i use qml Image element can only load the whole picture，i want select a part of the picture start at some pixels with specified width，how can i make it ture？

Any pointers whatsoever would be highly appreciated.
thanks