implementing stylesheet example px is not recognized

the line QRadioButton::indicator "indicator" does not name a type

Also where can the png files be found?

Here is the example

Code:

QRadioButton::indicator
{
    width: 13px;
    height: 13px;
}
 
QRadioButton::indicator::unchecked
{
   image: url(:/images/radiobutton_unchecked.png);
}
 
QRadioButton::indicator:unchecked:hover
{
    image: url(:/images/radiobutton_unchecked_hover.png);
}
 
QRadioButton::indicator:unchecked:pressed
{
    image: url(:/images/radiobutton_unchecked_pressed.png);
}
 
QRadioButton::indicator::checked
{
    image: url(:/images/radiobutton_checked.png);
}
 
QRadioButton::indicator:checked:hover
{
    image: url(:/images/radiobutton_checked_hover.png);
}
 
QRadioButton::indicator:checked:pressed
{
    image: url(:/images/radiobutton_checked_pressed.png);
}