Mostly blogger use this image as intro to viewers.you can make your custom header image in photoshop or in other photo editing tool or software.
center the header image in blogger
Now i tell you to set image in the middle is very easy just follow me in few steps.
First you have to login
Go to template and click customize
Now go to advance , you can see at the bottom of left side
click add css and paste the below code there.
click apply to the blog and view your blog to confirm that you image came in the middle.
Till if you have any problem feel free to ask i try my best to solve your problem.