Twitter redesign the site and also offer some new features.The new design maybe affect your old twitter background because it has different dimensions.
Tips for creating your own new twitter background
1.New design uses a fluid layout.
2.40 pixels top toolbar
3.20 pixels space between toolbar and content.
4.semi-opaque right sidebar and varying width.
5.540 pixels timeline column.
smaller than 800k. GIF, JPG, PNG
10 X 110 pixels width from left side if your are using 1280×800 screen resolution.
screen resolution can impact the width of the columns.
1.Log in to twitter
4.Select a template or upload your own background image by clicking Change background image.
5.Save the changes.