If you upload the image background from the page elements, you will notice that it will affect your mobile template, except you add the header tag code to your template html and exclude the title and description to avoid overlapping of the background image and title.
How to Change Your Blogger Header
- Login to your blogger dashboard
- Click on "Design" > "Edit HTML" > back up your template
- Use ctrl F to find the code below
- Replace it with the code below
- Now search for the following code
- Copy and paste the following code below the code above
After placement of code in the second search, your final result should look like the image below
- Replace BACKGROUND IMAGE URL with the url/link of your background/header image, if you don't know where to host your image, you may read how to host image on blogspot
- Edit width:600px; and height:100px; to adjust your background image size until it best suit your header size and make sure you preview your changes before you finally click "SAVE TEMPLATE"