Sometime back, just like Google+ and Facebook, Twitter also released the feature to add a big cover or your can say header image that shows up on your Twitter Profile. In this tutorial we will be discussing about the websites that allows you find cool Twitter headers image or you can them manually in your style using Twitter header image generator online tools.
Twitterheaders.co
Twitcover.com
Twitbacks.com
So these are some good services you can use for creating or downloading header images for your Twitter profile.
OR Alternatively you can use any other application from simple paint to photoshop etc for creating a header image for your Twitter profile by keeping its size in your mind. You should know that Twitter recommends 1200px × 600px image for header which gets resized to 520px × 260px.