Outra forma de fazer.

Cheguei no mesmo resultado desse jeito. Está correto?

label[for="nome"]{
    position: relative;
}

label[for="nome"]:after{
    background-color: #851944;
    background-repeat: no-repeat;
content:"";
background-image: url(../img/usuario.png);
background-size: 50% 50%;
width: 2em;
height: 2em;
background-position: center;
position: absolute;
top: 100%;
left: 0;
}

#nome{
    width: 36em;
    border-color: lightgrey;
    position: relative;
    left: 2em;
}




label[for="email"]{
    position: relative;
}

label[for="email"]:after{
background-color: #851944;
background-repeat: no-repeat;
content:"";
background-image: url(../img/email.png);
background-size: 50% 50%;
width: 2em;
height: 2em;
background-position: center;
position: absolute;
top: 100%;
left: 0;
}

#email{
width: 36em;
border-color: lightgrey;
position: relative;
left: 2em;
}