.s1::before,.s2::before,.s3::before {content:"";
background-repeat:no-repeat;
background-size:40px 20px;
display:block;
width:40px;
height:20px;
}
.s1::before {content: url("i-einfach.png")}
.s2::before {content: url("i-mittel.png")}
.s3::before {content: url("i-schwierig.png")}