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 . 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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