Setting sourceSize in Image can cause a grid-like pattern (compression artefact?) and blurriness. Attached is the output (magnified 4x) of the following minimal example with and without sourceSize. The image file (3840x2400 pixels) can be found at https://interfacelift.com/wallpaper/details/3162/christmas_eve_in_seattle.html

This bug causes bad rendering of wallpapers in KDE Plasma: https://bugs.kde.org/show_bug.cgi?id=338506

import QtQuick 2.5

Image {
	width: 1920    // EDIT: actually the aspect ratio needs to change for the bug to be triggered, so change this to 1800 for example
	height: 1200
	source: "03162_christmaseveinseattle_3840x2400.jpg"
	sourceSize: Qt.size(width, height)
}

EDIT: In fact this comes up only if sourceSize has a different aspect ratio than the original image. The aspect ratio should not matter though because the documentation says the source aspect ratio will not be altered.