<?php
$klasor=$_POST["klasor"];
?>
<form name="resim" action="#" method="post">

secenek 1 <input type="radio" name="klasor" value="a"/><br>
secenek 2 <input type="radio" name="klasor" value="b"/><br>
secenek 3 <input type="radio" name="klasor" value="c"/><br>
<br><br><input type="submit" value="getir"/>

</form>

<?
if ($klasor!="")
{
$resim=rand(1,50); //kaç tane resim varsa her klasörde;
echo "<img src=".$klasor."/".$resim.".jpg>";
}

?>