How does Twitter determine its homepage background picture?

Every time when we open Twitter using web browser, we may see a different homepage background picture. Sometimes it's a night view of a city, sometimes it's a picture about some baseball players. How does Twitter determine which picture to show at a particular time? Any logic behind this? Or it's just a random algorithm?

+1 (1 person wants to ask the same question) Posted at 2012-11-28 23:23:02