diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index cb6122a6aa4d50f1ce5844cea4dc5e5cc778848d..6ec4ef9a7bf56efe3f9fc4dc759869d16898f6e3 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -1122,6 +1122,11 @@ function template_preprocess_image(&$variables) {
 
   foreach (array('width', 'height', 'alt', 'title') as $key) {
     if (isset($variables[$key])) {
+      // If the property has already been defined in the attributes,
+      // do not override, including NULL.
+      if (array_key_exists($key, $variables['attributes'])) {
+        continue;
+      }
       $variables['attributes'][$key] = $variables[$key];
     }
   }
diff --git a/core/modules/image/src/Tests/ImageThemeFunctionTest.php b/core/modules/image/src/Tests/ImageThemeFunctionTest.php
index 97c5a2b1deb6dce9a2ef359f3a32e39c2414bd2b..88358398ae16698ed7e431d31bf1d0e185488ac9 100644
--- a/core/modules/image/src/Tests/ImageThemeFunctionTest.php
+++ b/core/modules/image/src/Tests/ImageThemeFunctionTest.php
@@ -152,4 +152,60 @@ function testImageStyleTheme() {
     $this->assertEqual(count($elements), 1, 'theme_image_style() renders an image correctly with a NULL value for the alt option.');
   }
 
+  /**
+   * Tests image alt attribute functionality.
+   */
+  function testImageAltFunctionality() {
+    // Test using alt directly with alt attribute.
+    $image_with_alt_property = array(
+      '#theme' => 'image',
+      '#uri' => '/core/themes/bartik/logo.png',
+      '#alt' => 'Regular alt',
+      '#title' => 'Test title',
+      '#width' => '50%',
+      '#height' => '50%',
+      '#attributes' => array('class' => 'image-with-regular-alt', 'id' => 'my-img'),
+    );
+
+    $this->drupalSetContent(drupal_render($image_with_alt_property));
+    $elements = $this->xpath('//img[contains(@class, class) and contains(@alt, :alt)]', array(":class" => "image-with-regular-alt", ":alt" => "Regular alt"));
+    $this->assertEqual(count($elements), 1, 'Regular alt displays correctly');
+
+    // Test using alt attribute inside attributes.
+    $image_with_alt_attribute_alt_attribute = array(
+      '#theme' => 'image',
+      '#uri' => '/core/themes/bartik/logo.png',
+      '#width' => '50%',
+      '#height' => '50%',
+      '#attributes' => array(
+        'class' => 'image-with-attribute-alt',
+        'id' => 'my-img',
+        'title' => 'New test title',
+        'alt' => 'Attribute alt',
+      ),
+    );
+
+    $this->drupalSetContent(drupal_render($image_with_alt_attribute_alt_attribute));
+    $elements = $this->xpath('//img[contains(@class, class) and contains(@alt, :alt)]', array(":class" => "image-with-attribute-alt", ":alt" => "Attribute alt"));
+    $this->assertEqual(count($elements), 1, 'Attribute alt displays correctly');
+
+    // Test using alt attribute as property and inside attributes.
+    $image_with_alt_attribute_both = array(
+      '#theme' => 'image',
+      '#uri' => '/core/themes/bartik/logo.png',
+      '#width' => '50%',
+      '#height' => '50%',
+      '#alt' => 'Kitten sustainable',
+      '#attributes' => array(
+        'class' => 'image-with-attribute-alt',
+        'id' => 'my-img',
+        'title' => 'New test title',
+        'alt' => 'Attribute alt',
+      ),
+    );
+
+    $this->drupalSetContent(drupal_render($image_with_alt_attribute_both));
+    $elements = $this->xpath('//img[contains(@class, class) and contains(@alt, :alt)]', array(":class" => "image-with-attribute-alt", ":alt" => "Attribute alt"));
+    $this->assertEqual(count($elements), 1, 'Attribute alt overrides alt property if both set.');
+  }
 }