There are a few points to keep in mind when creating your Twitter background image.
1. Real Estate -- The center of your image will be obscured by the actual content on Twitter, so putting elements of importance in the middle will not be optimal, as it'll be partially, if not completely, covered by all the juicy, can't-miss tweets.
2. Resolution -- Monitors of different resolutions will display background Twitter images differently. Creating images too small will cause abrupt cut-offs in your design or the need to use undesired tiling of your image on monitors of higher resolution. A good practice to remedy this problem is to create your image large enough so that it'll work on monitors of large resolution and by keeping elements of importance contained approximately in the upper leftmost area of the image. Note, that background images will be nearly unidentifiable on monitors of 800x600 resolution, as the twitter content covers most of the screen. And also, if you're still using a monitor at 800x600 resolution... duuude.
3. Alignment -- It's also worth noting that your background image will be aligned to the LEFT of a viewer's screen. If you create an image suitable for large resolution monitors, remember that any excess of your image will be cut off on the right and bottom on monitors of smaller resolution. Many people choose to create background images which fade to a solid color on the right to prevent undesired cropping of background images or unwanted tiling.
As I mentioned, it's good to begin with a larger image to display on monitors of varying resolutions. I find starting with a canvas size of 1680x1200 pixels covers the majority of resolutions out there.
Remember to keep space available at the top of your design to incorporate the Twitter logo and navigation bar.
Also, a lot of people choose to include a sidebar on the left. This allows you to include additional information about yourself including links to personal sites, your email address, or pictures of your dog that nobody cares to see. Again, it's worth mentioning that you're probably going to want the sidebar to be viewable on monitors of varying resolution. A good way to ensure this is to try to keep your sidebar around 200pixels wide, so as not to be obscured by the content in the middle on smaller monitors.
Here are some examples of Twitter backgrounds that can be created in Aviary.
Feel free to open up any of the creations to see how they were made.
Create your own twitter backgrounds and post 'em here! Let's see what you're doing!
Having troubles getting started? Check out this tutorial on creating Twitter backgrounds from start to finish!]]