Format the Layout of Images In The Edit Post Textarea?

Add this to your theme’s functions.php file:


add_editor_style();


By default that function will load a file called editor-style.css which is located in the root directory of your theme. The functions accepts a filename or an array of filenames as parameter. Reference in the Codex:
http://codex.wordpress.org/Function_Reference/add_editor_style

If you want (or need to have) more control over the custom CSS file name and location you can use this function instead:

function custom_editor_style($url) {

  if ( !empty($url) )
    $url .= ',';

  // Change the path here if using sub-directory
  $url .= trailingslashit( get_stylesheet_directory_uri() ) . 'editor-style.css';

  return $url;
}
add_filter('mce_css', 'custom_editor_style');