Extract image src from a post and send it to an external form

Extract image src from a post and send it to an external form

I have created an Facebook App to upload photos to facebook from image url with help of some tutorials. It needs an image url and description. I want to put a button "Upload to Facebook" below every "Image" type posts in wordpress.
App Main Part to upload images -
<?php
if(isset($_POST["source"]))
{
try {
    $access_token=$facebook->getAccessToken();
    $graph_url= https://graph.facebook.com/me/photos?
  . "url=" . urlencode($_POST["source"])
  . "&message=" . urlencode($_POST['message'])
  . "&method=POST"
  . "&access_token=" .$access_token;
    $response=file_get_contents($graph_url);
    $json=json_decode($response);
  }
  catch (FacebookApiException $e) {
    error_log('Could not post image to Facebook.');
  }
}
?>
    <form enctype="multipart/form-data" action=" " method="POST">
        Paste an image URL here:
        <input name="source" type="text"><br/><br/>
        Say something about this photo:
        <input name="message"
               type="text" value=""><br/><br/>
        <input type="submit" value="Upload" class="btn btn-primary"/><br/>
    </form>
How can i extract image src dynamically from a custom post type(Image), and set src as source in form automatically. (There is only one image in every image type post)