Firstly chose an image than can be fairly long and thin.
I use Preview on my Apple laptop to edit the photo because it is standard on all Apple laptops.
Use the Tool Menu to crop your original photo.
Now use the tool menu again to adjust the size to about 1000px wide. Don't worry about the height.
Save the resized image to your desktop so you can easily find it.
Go to your OVERVIEW and go to LAYOUT.
Look where it says EDIT THE HEADER
Upload your cropped and resized photo.
Go back to the OVERVIEW. Click on TEMPLATE, then CUSTOMISE and ADJUST THE WIDTH until you get it just right.
When you do APPLY IT TO YOUR BLOG.