This helps to fix: https://github.com/magento/magento2/issues/25099

imageuploadfix.diff

--- a/vendor/magento/module-catalog/Model/Category/Attribute/Backend/Image.php	2020-01-02 11:51:58.694620443 +0100
+++ b/vendor/magento/module-catalog/Model/Category/Attribute/Backend/Image.php	2020-01-02 12:06:08.734544971 +0100
@@ -125,7 +125,9 @@
         }
 
         if ($imageName = $this->getUploadedImageName($value)) {
-            $imageName = $this->checkUniqueImageName($imageName);
+            if (array_key_exists('tmp_name', $value[0])) {
+                $imageName = $this->checkUniqueImageName($imageName);
+            }
             $object->setData($this->additionalData . $attributeName, $value);
             $object->setData($attributeName, $imageName);
         } elseif (!is_string($value)) {