You might want to rewrite you code a little like this
$body = "
<html>
<head>
<title>Untitled Document</title>
</head><body>
<img src=\"http://www.domain.com/images/emailheader_.jpg\"><br
I got some message here
</body></html>"
;
$from="myemail@mail.com";
$to = "received@mail.com";
$headers = "MIME-Version: 1.0\r\n";
$headers .= "Content-type: text/html; charset=iso-8859-1\r\n";
$headers .= "From: $from \r\n";
mail($to,"wtf test",$body,$headers);
Your code should work with the full URL to the image emailheader_.jpg in this case You should get the email with just the blank image if the URL to the image is wrong (or you are not online while reading), Or you could read the image into a file and then use an inline attachment. This is rather complex and takes a multipart/mixed content type plus you need to encode using base64 encoding and use boundarys between the attachment and the message itself, and using this method you would then just use the name of the file that has been read and encoded, not the full URL just the name of the file emailheader_.jpg