Before this,first take a backup of your blog template,so there is a chance to get back to your current template if you made any mistakes.
1.First sign in to your blogger account>Edit html


2.If you have photoshop or any other tools,crop the picture corretly to 660 px(width).If you don't have photoshop,then you can crop a photo by this free online tool Click here.
3.So, your image is ready now! click the page elements tab again.

Click on the edit button as shown below next to your blog name.

4.And you will go to a window just like the below one.First select,the option whether you want to place the image behind the title and description or insted of title and description.Next,browse for the image and save the changes after the image has uploaded.

5.That's it! Now, your blog has a background image in the header.
Have any questions?ask your question via comments.