Now that you have your image ready to be uploaded, go back to Appearance and click on Nirvana Settings. Under HEADER SETTINGS click on DEFINE HEADER IMAGE.
New window will pop up. Click on Choose File, than find and select the image that you just edit, and click upload.
Than you can either choose to Crop and Publish, or Skip Cropping, Publish Image as Is. Since you already have your image the right size, choose the second option.
Click on Save Changes and close this window.
Wile you still under HEADER SETTINGS section in the Nirvana Settings, make sure the boxes for Center the header image horizontally and Force header image responsiveness options are selected.
Now you can go back to your website and click the refresh button to see the changes.
Your next step will be to create a logo and upload it to your site.