Thank you so much for using our product. As you request, pls send the website information (URL and admin account) via email huyentt@vietbrain.com and our developer will check it for you.
Our DEV checked it and found out that issue was cause by https protocol. Basically, if you want to add more images, pls go to Content -> Media, select Image.
We will try to fix and update the new version shortly.