Start with a background image 2500 x 1600 is what I used to make sure it wouldn't tile. (hint you can visit www.morguefile.com to get free background pics like the one above).
Create your insert by making a new transparent image 339 x 663 write whatever you want on that insert.
Merge the two pictures together. Make sure to keep the insert .5 away from the top and left for a cleaner image. Save the completed picture as a jpg to make upload faster. Really there's nothing to this and with a little trial and error you can have a great facebook landing page.
After you make your background image: Go to your twitter profile. Select Design. Then upload the new background. That's it your done. You can mess around with the background color and links until it matches your new background image!
You can see my profile on twitter at www.twitter.com/caradonovan_ If you get stuck just leave me a message below and I'll help. Don't forget to share your new background with me here! I love seeing what people create!