core/modules/system/templates/image.html.twig      | 5 +++--
 core/themes/classy/templates/field/image.html.twig | 5 +++--
 core/themes/stable/stable.theme                    | 6 ++++--
 core/themes/stable/templates/field/image.html.twig | 5 +++--
 4 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/core/modules/system/templates/image.html.twig b/core/modules/system/templates/image.html.twig
index 4ca403f..bdc0b00 100644
--- a/core/modules/system/templates/image.html.twig
+++ b/core/modules/system/templates/image.html.twig
@@ -22,8 +22,9 @@
  * - title: The title text is displayed when the image is hovered in some
  *   popular browsers.
  * - attributes: HTML attributes for the img tag.
- * - srcset: Array of multiple URIs and sizes/multipliers.
- * - sizes: The sizes attribute for viewport-based selection of images.
+ * - srcset: (optional) Array of multiple URIs and sizes/multipliers.
+ * - sizes: (optional) The sizes attribute for viewport-based selection of
+     images.
  *   - http://www.whatwg.org/specs/web-apps/current-work/multipage/embedded-content.html#introduction-3:viewport-based-selection-2
  * - style_name: (optional) The name of the image style applied.
  *
diff --git a/core/themes/classy/templates/field/image.html.twig b/core/themes/classy/templates/field/image.html.twig
index 690e0d8..b71a3e3 100644
--- a/core/themes/classy/templates/field/image.html.twig
+++ b/core/themes/classy/templates/field/image.html.twig
@@ -23,8 +23,9 @@
  * - title: The title text is displayed when the image is hovered in some
  *   popular browsers.
  * - attributes: HTML attributes for the img tag.
- * - srcset: Array of multiple URIs and sizes/multipliers.
- * - sizes: The sizes attribute for viewport-based selection of images.
+ * - srcset: (optional) Array of multiple URIs and sizes/multipliers.
+ * - sizes: (optional) The sizes attribute for viewport-based selection of
+     images.
  *   - http://www.whatwg.org/specs/web-apps/current-work/multipage/embedded-content.html#introduction-3:viewport-based-selection-2
  * - style_name: (optional) The name of the image style applied.
  */
diff --git a/core/themes/stable/stable.theme b/core/themes/stable/stable.theme
index 9af2d6e..dd8730b 100644
--- a/core/themes/stable/stable.theme
+++ b/core/themes/stable/stable.theme
@@ -49,9 +49,11 @@ function stable_preprocess_links(&$variables) {
  *   - title: The title text is displayed when the image is hovered in some
  *     popular browsers.
  *   - attributes: Associative array of attributes to be placed in the img tag.
- *   - srcset: Array of multiple URIs and sizes/multipliers.
- *   - sizes: The sizes attribute for viewport-based selection of images.
+ *   - srcset: (optional) Array of multiple URIs and sizes/multipliers.
+ *   - sizes: (optional) The sizes attribute for viewport-based selection of
+ *     images.
  *     - http://www.whatwg.org/specs/web-apps/current-work/multipage/embedded-content.html#introduction-3:viewport-based-selection-2
+ *  - style_name: (optional) The name of the image style applied.
  */
 function stable_preprocess_image(&$variables) {
   if (!empty($variables['uri'])) {
diff --git a/core/themes/stable/templates/field/image.html.twig b/core/themes/stable/templates/field/image.html.twig
index 8bdf886..b79450b 100644
--- a/core/themes/stable/templates/field/image.html.twig
+++ b/core/themes/stable/templates/field/image.html.twig
@@ -22,8 +22,9 @@
  * - title: The title text is displayed when the image is hovered in some
  *   popular browsers.
  * - attributes: HTML attributes for the img tag.
- * - srcset: Array of multiple URIs and sizes/multipliers.
- * - sizes: The sizes attribute for viewport-based selection of images.
+ * - srcset: (optional) Array of multiple URIs and sizes/multipliers.
+ * - sizes: (optional) The sizes attribute for viewport-based selection of
+     images.
  *   - http://www.whatwg.org/specs/web-apps/current-work/multipage/embedded-content.html#introduction-3:viewport-based-selection-2
  * - style_name: (optional) The name of the image style applied.
  *