If you're visiting Twitter using your computer, you can upload your cover by clicking the image of the cog in the upper right-hand corner and choosing to “Edit Profile.” Choose “Design” from the left-hand menu, and then scroll down to the “Customize Your Own” section.
You will find some of the best Twitter Covers available on the internet from TwitrCovers. All Twitter covers are divided into categories like Nature, Movies, Animals, Art, Games, Quotes and etc. I am sure you can pick few ones for yourself.

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