On a lot of Twitter feeds you will see a image to the left of the feed itself which makes it look as if the Twitter page itself has been designed. Basically what people are doing is adding a background image and typically using the tile feature to tile the image across the background. One thing to note is the fact that the image will display differently for various screen resolutions.
If you notice in the below image the background image fits perfectly for the screen resolution the page was viewed in which was 1280×800. The background image uploaded in this example below has a width of 250 pixels and a height of 600 pixels. Again these deminsions will make the image fit perfectly in a browser fully expanded when shown on a monitor/screen with a resolution of 1280×800.