You can update the images in the header of your blog by doing the following...
First prepare your image. We recommend resizing it and cropping it to the correct size before uploading it, (1900x900 is the recommended size for version 2 for the header or 790px by 190px for version 1).
Log in to your website admin.
Go to Website Content icon.
Click on Upload Photo using Photo Editor.
Upload your photo (note the filename).
After uploading the photo, return to the Main Menu.
Click on My Blog.
Login to your Blog.
Click Customization Settings on the right column.
In the Header Image or Logo image field, select the image name you uploaded in the admin.
Your image should now show in the header of your Blog.
Click to Save Changes.