including captcha

hi
i need to include a captcha in a form, the code is a simple captcha wich goes on like this:
<?php
session_start();

//Now lets use md5 to generate a totally random string
$md5 = md5(microtime() * mktime());

/*
We dont need a 32 character long string so we trim it down to 5
*/
$string = substr($md5,0,5);


/*
Now for the GD stuff, for ease of use lets create
the image from a background image.
*/

$captcha = imagecreatefrompng("./CAPTCHA.png");

/*
Lets set the colours, the colour $line is used to generate lines.
Using a blue misty colours. The colour codes are in RGB
*/

$black = imagecolorallocate($captcha,221, 36, 175);
$line = imagecolorallocate($captcha,233,239,239);;

/*
Now to make it a little bit harder for any bots to break,
assuming they can break it so far. Lets add some lines
in (static lines) to attempt to make the bots life a little harder
*/
imageline($captcha,0,0,39,29,$line);
imageline($captcha,40,0,64,29,$line);

/*
Now for the all important writing of the randomly generated string to the image.
*/
imagestring($captcha, 5, 20, 10, $string, $black);


/*
Encrypt and store the key inside of a session
*/

$_SESSION['key'] = md5($string);

/*
Output the image
*/
header("Content-type: image/png");
imagepng($captcha);

?>

it works when i test it on a single page but when i include it on my web page i have a problem with the headers already sent stuff:
<label for="saisir_code">
Veuillez saisir le code <SUP class="oblig">*</SUP><br>
<input type="text" name="saisir_code" id="saisir_code" class="text">
</label>
</td>
<td style="padding-left: 100px">
<?php

$md5 = md5(microtime() * mktime());
$string = substr($md5,0,5);
$captcha = imagecreatefrompng("includes/images/CAPTCHA.png");

$black = imagecolorallocate($captcha,221, 36, 175);
$line = imagecolorallocate($captcha,233,239,239);;
imageline($captcha,0,0,39,29,$line);
imageline($captcha,40,0,64,29,$line);
imagestring($captcha, 5, 20, 10, $string, $black);
header("Content-type: image/png");
imagepng($captcha);

?>
</td>
<td>
</td>
</tr>

<tr>
so i get this alreday sent headers error , i cannot see how to place the imagepng($captcha); code because if i put it on top of page the whole page will be a png.



Warning: Cannot modify header information - headers already sent by (output started at C:\Program Files\xampp\htdocs\my\www3\includes\_top.php:23) in C:\Program Files\xampp\htdocs\my\www3\includes\_myin.php on line 222
‰PNG IHDR‹ _¬cAPLTEÿÿÿÝ$¯éïïRˆáIDAT8��ÔA Ð^ÂxïÐÍÐd®Ù˜ô°hâ�œÂK°qï=”Ø�áSgEÂcþ´4íÝReõ[ZÊR½ún/ë#4»ô‡—õ€MÙJД-‹�„��²‘00P6Ö2–°‘ó9_úkrS$ãL@&‡å]A#“Ã"“w¯O;dÎAMLÊYÂ}Nš™�š. ÎCáùAÞ‡[£OŒ&Ntu:áyÂ@ÌÙXl4'Þzv¾�[ï�äÎjW/f¹³Ô2ób[fN©\}1sJÛŒëvÀëvÀÀêáÿdUo/mŒ$6�IEND®B`‚

http://forums.codewalkers.com/showpost.php?p=39902&postcount=3

I should also have included this text from the tutorial (http://www.codewalkers.com/c/a/Miscellaneous/Creating-a-CAPTCHA-with-PHP/4/) you likely followed:
Then you just display the image to the user with a simple <img xsrc="captcha.php" border="0"> tag.

hi, thanks for your reply, so i also put the header("Content-type: image/png"); in the imaege.php file?
regards.

That's right, because that's the content you want delivered as an image.