they are images saved to the images/Icons/ folder, for example the details button uses:
/images/Icons/view_details.png
.formButton.ResultsPageButton.DetailButton {
background-image: url("../images/Icons/view_details.png");
background-position: center center;
background-repeat: no-repeat;
cursor: pointer;
height: 29px;
width: 29px;
}