Código PHP:
<form name="personas" method="post" action="" >
<select name="Combo_personas" id="Combo_personas" onchange="document.personas.submit();">
<option value="">Seleccione</option>
<?php
while($fila = mysql_fetch_array($result)){
?>
<option value="" <?php if ($_POST["combo_personas"]== $fila["personas"]) echo "selected" ?>><?php echo $fila["personas"]?></option>
<?php
}
mysql_free_result($result);
?>
</select>
<?php
$result1=mysql_query("select * from img_redes where img_personas='".$_POST["combo_personas"]."'");
?>