Jafor Ahmed

The image is added to the background of the class .site-header

You can use the below CSS if you don’t want any image in the header background.
.site-header{
background: none;
}

If you add the code into the Additional CSS of WordPress, it will remove header from each and every page which uses the same class for the background. https://prnt.sc/ugojyu

It seems like you cached your admin area also. Try to access the site via FTP rename the caching plugin. I also noticed that you are using CloudFlare Firewall. Whitelist your IP from CloudFlare and/or deeply check the Firewall rules. Check this CloudFlare Doc for editing creating or deleting a rule.