Plugin option default value

There are two errors in your code.

1. "echo'plugins_url( '/image/top.png', __FILE__ )'" is not executed, it is just a string that happens to look like PHP code.
   What you really want is:

   $arr = array( "top_image"=> plugins_url( '/image/top.png', __FILE__ ) );
   

2. Do not store default options in the database. Use the second parameter for get_option( $option, $default ); instead.

   So whenever you fetch the option, provide the default values as second argument:

   $option = get_option( 
       'scroll_options', 
       array( 
           "top_image"=> plugins_url( '/image/top.png', __FILE__ ) 
       ) 
   );
   

   The reason is that you cannot rely on the stored values anyway. They might be deleted already, so you have to provide the default values always.