Hi,
You can quickly check if any plugin is conflicting by disabling all plugins and try uploading header image again, enable plugins one by one to check the conflict and update plugins as necessary.
How to upload header image:
I could see on your site that you have uploaded the logo already, so, are you seeing any error when you upload a bigger header image? as the uploader is the same for both logo and header.
If issue still persists, send login details to [email protected] and I will check it out.