Use HTML 'center' tag. Inside the center tag, upload image using their default 'upload image'. It automatically displayed in center.

If the image is wide enough then it will cover the max width of the content area and there is no need to align the image in center. This is the best way I have found till now. Nothing else works.

All images in articles/posts are centered by default.

So far the way I found is to use <img src="" style="display: block; margin: 0 auto;" /> for images instead of markdown notation ![](image-link).