How to edit blockquote image

You can use add_editor_style() to override the default styles with your custom ones. Like this:

function wpse158918_override_default_tinymce_styles() {  
    add_editor_style( 'your-editor-styles.css' );
add_action( 'after_setup_theme', 'wpse158918_override_default_tinymce_styles' );

The code would go into your functions.php, the path to the stylesheet is relative to the directory of your current theme. If you are not doing this from your theme, but a plugin, then use the mce_css filter. Example from codex page:

function plugin_mce_css( $mce_css ) {
    if ( ! empty( $mce_css ) )
        $mce_css .= ',';

    $mce_css .= plugins_url( 'editor.css', __FILE__ );

    return $mce_css;
}
add_filter( 'mce_css', 'plugin_mce_css' );