The way I did it, example:

TreeView {
id: treeview
anchors.fill: parent
model: treemodel
style: TreeViewStyle {
branchDelegate: Rectangle {
width: 15; height: 15
Image {
source: styleData.isExpanded ? "images/16px/ArrowDownSmall_black_16px.svg" : "images/16px/ArrowRightSmall_black_16px.svg"
}
}

Simply add a branchDelegate, then in my example I checked isExpanded and put my own images like the example.