1. Open your favorite image editing program (I used gimp 2) and create an image that is 1920 x 1080 with 72 DPI.
2. Create the image that you want to be your background. Any information that you want to display should be placed on the left, top side no more than 225 pixels away from the edge.
3. Save this image as a .jpg.
4. Go to twitter and click on the gear in the top right corner. Then click on "Design." (see pictures below)
5. Click on "Change background" and upload your photo.
6. Click save changes and you're done!