ENGCOM-2898: [Backport] Fixes black background for png images in wysiwyg editors. #17855

 - Merge Pull Request #17855 from eduard13/magento2:2.1-develop-bp-16733
 - Merged commits:
   1. 99eb521

Author: magento-engcom-team

lib/internal/Magento/Framework/Image/Adapter/Gd2.php

@@ -63,6 +63,16 @@ public function open($filename)
         }
         $this->imageDestroy();
         $this->_imageHandler = call_user_func($this->_getCallback('create'), $this->_fileName);
+        $fileType = $this->getImageType();
+        if (in_array($fileType, [IMAGETYPE_PNG, IMAGETYPE_GIF])) {
+            $this->_keepTransparency = true;
+            if ($this->_imageHandler) {
+                $isAlpha = $this->checkAlpha($this->_fileName);
+                if ($isAlpha) {
+                    $this->_fillBackgroundColor($this->_imageHandler);
+                }
+            }
+        }
     }
 
     /**