per centrare verticalmente devi dare una regola all'immagine
che altrimenti seguirebbe la "baseline"
codice HTML:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" 
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>prova</title>
<style type="text/css">
#menu {
    float: left;
    width: 250px;
    background: red;
    padding-left: 0px;
    padding-top: 10px;
}
.lista_menu {
    list-style-type: none;
    padding-left: 20px;
    margin: 0px;
    line-height: 100px;
    padding-bottom: 10px;
}
.lista_menu a img {
    height: 100px;
    vertical-align: middle;
    border: none
}
</style>
</head>
<body>
<div id="menu">
  <ul class="lista_menu">
    <li><a href="a.html"><img src="1.png">a</a></li>
    <li><a href="a.html"><img src="1.png">a</a></li>
    <li><a href="a.html"><img src="1.png">a</a></li>
    <li><a href="a.html"><img src="1.png">a</a></li>
    <li><a href="a.html"><img src="1.png">a</a></li>
  </ul>
</div>
</body>
</html>