Like what you see when you log in to Twitter? Great, then you don't need to read this article! For those who may prefer a more personal look and feel when browsing the site, we've given you the option to customize your profile. When you design your profile page, everything you see on Twitter will be set in the theme you have chosen. Not only are you making yourself unique to those viewing your profile, you are creating a customized experience for yourself on Twitter.
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Note: Background images must be in PNG, GIF, or JPG format, smaller than 800k. If your image's file size is too large, the easiest way to get it down is to use and image editing program or web app to shrink the image. Animated GIFs are not supported as background images.
If you're having trouble uploading an image as your background, click here for image-upload troubleshooting tips.