Now choose File>New. Set a width and height appropriate for the design you have in mind; for the toy example, I've chosen a width of 350 and a height of 500 pixels. Ensure that Background Contents is set to Transparent and then click OK to create the file. You can certainly make your design taller or wider; just keep in mind that the focal point of the design is at the top left, adjacent to your most recent tweet.
Now paste the masked image into the document (Cmd/Ctrl-P). Resize, rotate, and position the image as you wish. Avoid centring the object within the document; instead, offset it to the left so that part of it is out of view. Once the object is in place, you can incorporate text or graphic elements to give the background a bit more flair.
When you're done, follow these steps to export your image:
* Choose File>Save for Web & Devices.
* Choose PNG-24 from the Preset pop-up menu.
* Ensure that Transparency and Convert to sRGB are both checked.
* Ensure that the projected file size (seen in the bottom right) is less than 800k.
* Click Save and save the file to your Desktop.
Getting your design online
Go to your Twitter home page and, from the top right menu, click Settings. Twitter will present you with a series of tabs; to continue, click Design. In this section, you'll find Twitter's selection of ho-hum themes. Right below that, you'll find two important links: Change background image and Change design colors.
Start by clicking Change background image. Click Choose File and find the picture you just saved to your Desktop; once you've selected it, click Choose. (Don't check the option to tile the background.) Click Save Changes to confirm the change.
Now you'll need to tweak the page colours. Click Change design colours and then click the colour block under Background. Twitter's colour picker will appear. Choose any colour and you'll see that the background of your page is instantly updated. As long as you've masked your image well, you should be able to choose any colour background; experiment with different hues until you find one that really complements your image. Once you've found the right color, click Done. You can then set custom colours for your text, links, sidebar and sidebar border; to finish up, click Save Changes.
And that's it. The new design is locked in -- now it's time to tweet about it!