The new sourceClipRect property for Image, introduced in 5.15.0, does not work for "image:" urls or texture files.

In QQuickPixmap::load(), whether synchronous (createPixmapDataSync()) or not (QQuickPixmapReader::processJob()), there end up being three possible paths the image can be loaded through:

1. If it has an image: url, the image is loaded through the image provider
2. Otherwise, if QTextureFileReader::canRead() the url, it is loaded through a QSGTextureReader

3. Otherwise, it is loaded through QImageReader::read()
    

In the third case here, QImageReader::read() applies the sourceClipRect by copy()ing a portion of the image, if necessary (its handler might also handle the clipping itself). But in the other two cases, the QQuickPixmap resolves with the full (unclipped) image.
 
As a workaround, the Image item can be wrapped in another item with clip: true. Then the width and height of the wrapper can be set to the desired clip rect size, and the x: and y: can be set to the negative of the clip rect origin.