Issue #1828924 by pfrenssen: Convert the Image Style Preview Image to CMI.

core/modules/image/config/image.settings.yml

+preview_image: core/modules/image/sample.png

core/modules/image/image.admin.inc

@@ -697,7 +697,7 @@ function theme_image_style_effects($variables) {
 function theme_image_style_preview($variables) {
   $style = $variables['style'];
 
-  $sample_image = variable_get('image_style_preview_image', drupal_get_path('module', 'image') . '/sample.png');
+  $sample_image = config('image.settings')->get('preview_image');
   $sample_width = 160;
   $sample_height = 160;
 

core/modules/image/image.install

@@ -167,3 +167,14 @@ function image_update_8001() {
   db_drop_table('image_styles');
   db_drop_table('image_effects');
 }
+
+/**
+ * Moves image module settings from variable to config.
+ *
+ * @ingroup config_upgrade
+ */
+function image_update_8002() {
+  update_variables_to_config('image.settings', array(
+    'image_style_preview_image' => 'preview_image',
+  ));
+}