Key Points on creating banners:
-First of all, original photograph of the banner must be free to use. We can not use a licensed photo to create banners. If picture has one of these licenses, it's free to use:
Creative Commons — Attribution 3.0 Unported — CC BY 3.0
Creative Commons — Attribution 2.0 Generic — CC BY 2.0
And if it has a reserved license, We can not use it as a banner.
You can easily find out it in license sections of Flickr, wikipedia, wikimapia etc.
Burj Khalifa Park #2 | Flickr - Photo Sharing! (Look at license section on right below of the page.)
New York New York | Flickr - Photo Sharing! (A licensed picture.)License
Attribution Some rights reserved
-Picture must be original, in other words, we don't want to use a picture as a banner which is published on other forums.License
All Rights Reserved
-Dimensions for SSL Banners> 760*120 (That means 760px width - 120px height)
-Photo quality must be well.