Once the image is saved to your computer, you'll need to sign into your blogger account.  Click Customize, Click Layout, Click Edit Header (under Nav Bar).   You will want to install an image from your computer so select that option.  Now click Browse and find where you saved the image.  Under "Placement", click Instead of Title and Description.  Now save your changes and preview to make sure it's correct.  You may need to go back in and select "shrink to fit" if the banner appears to large.  If it's ok, click the orange/red SAVE box.  That's it!