To my surprise, the Image component has no radius property. I tried emulating the rounded corners by putting the image in a rounded Rectangle
Image
radius
Rectangle
If you have a unicolor background, you can draw with the border of a rounded rectangle on top.
Image{ id:img } Rectangle { // rounded corners for img anchors.fill: img color: "transparent" border.color: "blue" // color of background border.width: 4 radius: 4 }