To add an image to the header of blogger, first go to Blogger’s Dashboard > Design > Page Elements tab. Under this tab you will find the layout of the template you are using. At top you will find the Header widget, click Edit.
A pop up window will open. You will find that here you can change your title, description and image. Now upload your image from computer or put the URL of the image if you already had uploaded it. Check the box “shrink to fit” if you think that your image is greater in size than you header.
At last save your widget.