Hi @Lisbetti and Welcome,
AFAIK you will need some C++ API's as well for cropping the image. Once you grab the image using grapToImage call a C++ function and pass this image to it for further processing. Once you receive it on C++ side you have QImage::copy function to copy the desired part. This function too returns a QImage which you can send it to QML again for displaying.
Note: If you are a beginner C++ and QML interaction may sound a bit tedious. Pls refer the fine docs for further explanation: