bal, try using the WP built-in function to get the thumbnail version of the image. You can set the size of the thumbnail that WP will create when you upload the image on your post.
The code should look something like this:
//Get images attached to the post
$args = array(
'post_type' => 'attachment',
'post_mime_type' => 'image',
'numberposts' => -1,
'order' => 'ASC',
'post_status' => null,
'post_parent' => $post->ID
);
$attachments = get_posts($args);
if ($attachments) {
foreach ($attachments as $attachment) {
$img = wp_get_attachment_thumb_url( $attachment->ID );
echo '<link rel="image_src" href="'.$img.'">';
break;
}
Also check out this function, which could be used to pull the "medium" or "large" version of the uploaded image:
http://codex.wordpress.org/Function_Reference/wp_get_attachment_image_src