select

The default appearance of an option (i.e., an xf:item) within a select control is that of a check-box:

xf\:select .pc-selected
{
  background-image: url(check-selected.png);
}

xf\:select .pc-deselected
{
  background-image: url(check-deselected.png);
}

The images used are:

check-selected.png:   

check-deselected.png: