Function for uploading images from URL to WordPress. Thanks to Andrew Norcross for help with this.

gistfile1.phtml

<?php 
	function upload_external_image( $post_id, $img_url ) {

		// load media handlers
		require_once( ABSPATH . 'wp-admin' . '/includes/image.php' );
		require_once( ABSPATH . 'wp-admin' . '/includes/file.php' );
		require_once( ABSPATH . 'wp-admin' . '/includes/media.php' );

		// create a temp file
		$temp_file	= download_url( $img_url );

		// make a file array
		$file_array	= array(
			'name'		=> basename( $img_url ),
			'tmp_name'	=> $temp_file,
		);

		// Check for download errors
		if ( is_wp_error( $temp_file ) ) {
			@unlink( $file_array[ 'tmp_name' ] );
			return $temp_file;
		}

		// run sideload function
		$image_id = media_handle_sideload( $file_array, $post_id );

		// Check for handle sideload errors.
		if ( is_wp_error( $image_id ) ) {
			@unlink( $file_array['tmp_name'] );
			return $image_id;
		}

		// send back the image ID
		return $image_id;

	}
?>