As you might have seen that Twitter has released New Header Images called Twitter Covers for each user’s profile. You may add any image to create your own Header Image if you like.
Or I have found TwitrCovers, which has a good collection of Twitter Covers. TwitrCovers has divided them into categories like Abstract, Cars, Nature, Places, Movies, Quotes and etc.
To change the Twitter Cover on your profile, simply hover over any images you like and click “Download Now”. TwitrCovers.com takes you to the image page where you can download the image manually or click “Make This My Twitter Cover”. It will automatically change the Twitter cover image to your twitter profile.