Changing the header image should be fairly simple from the theme control in the admin panel. There are a couple of things you can double check.

1. Make sure all of the server requirements are met. Note php 7 does cause some issues since the platform was never cleared to fully support it. Server requirements are on the right hand side of this page.
https://developers.oxwall.com/hosting

2. Check this post to help check file / folder permissions.
https://developers.oxwall.com/forum/topic/14334

Hi Darryl, thank you for your support. I tried to follow your instructions but I guess I missed something in between. Anyway, kindly check the attachment as to the recommended PHP extensions and settings. I'm using x10Hosting.com and my current PHP Version is 5.5 but I'm not quite sure if the settings in my cPanel matched with the recommended ones, that's why I included herein the screenshots for your reference. Other info are the following

Database server

• Server: Localhost via UNIX socket
• Server type: MariaDB
• Server version: 10.0.31-MariaDB-cll-lve - MariaDB Server
• Protocol version: 10
• User: unorhs79@localhost
• Server charset: UTF-8 Unicode (utf8)

Web server

• cpsrvd 11.56.0.22
• Database client version: libmysql - 5.1.73
• PHP extension: mysqli 

phpMyAdmin

• Version information: 4.0.10.14
• Documentation
• Wiki
• Official Homepage
• Contribute
• Get support
• List of changes

I really appreciate your support on this matter.

I wouldn't be able to tell you if your server requirements are okay, or not. You could provide the Oxwall requirements to your host, and they can tell you if they are all met, or not.
From your other post regarding not being able to change the avatar; I would lean toward the folder, and file permissions. If the platform can't access the folders, it won't be able to add the image.

Thank you so much Darryl, I do really appreciate your time in answering questions here. More power!