To create an HTML email you can do something like this:
...
$message = "<html><head></head><body>";
$message .= "<img src='link-image.jpg' alt='' /></body></html>";
$headers = "From: $from_email";
$headers .= "Content-type: text/html";
mail($to, $subject, $message, $headers);
This should build an HTML email for you and you should then be able to insert just normal html.
edit
You can read more about how to create HTML emails using PHP from here: http://css-tricks.com/sending-nice-html-email-with-php/
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