Rectangle{
                    id:idRectRound
                    width: 250
                    height: 250
                    radius: width/2
                    anchors.centerIn: parent
                    color: "#ff00ff"
                    visible: false
                }
                Image {
                    id: idRectImg
                    width: 250
                    height: 250
                    anchors.centerIn: parent
                    source: "qrc:/res/demo.png"
                    visible: false
                    smooth: true
                }
                OpacityMask {
                    anchors.fill: idRectRound
                    source: idRectImg
                    maskSource: idRectRound
               }

import QtQuick 2.12
import QtQuick.Window 2.12
import QtGraphicalEffects 1.0
 
Window {
    width: 640
    height: 480
    visible: true
    title: qsTr("Hello World")
    color:"black"
    Rectangle{
                        id:idRectRound
                        width: 250
                        height: 250
                        radius: width/2
                        anchors.centerIn: parent
                        color: "#ff00ff"
                        visible: false
                        border.color: "yellow"
                        border.width: 2
                    }
                    Image {
                        id: idRectImg
                        anchors.centerIn: parent
                        source: "qrc:/res/demo.png"
                        visible: false
                        smooth: true
                    }
                    OpacityMask {
                        anchors.fill: idRectRound
                        source: idRectImg
                        maskSource: idRectRound
                   }
}

#include <QGuiApplication>
#include <QQmlApplicationEngine>
 
 
int main(int argc, char *argv[])
{
#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
#endif
    QGuiApplication app(argc, argv);
 
    QQmlApplicationEngine engine;
    const QUrl url(QStringLiteral("qrc:/main.qml"));
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
                     &app, [url](QObject *obj, const QUrl &objUrl) {
        if (!obj && url == objUrl)
            QCoreApplication::exit(-1);
    }, Qt::QueuedConnection);
    engine.load(url);
 
    return app.exec();
}