Twitter is known for its sub-140 character bursts of commentary, but there are a fair amount of photos that get passed around as well on the network.

How many? About 20 million a week, according to Skylines, an Amsterdam-based company that bills itself as the first real-time photo search engine. At Mashable's request, Skylines analyzed photos that circulated on Twitter over the seven-day period from Nov. 10 to Nov. 17. As you might expect, many of the most-retweeted photos came from pop stars and were merely a reflection of those stars' notoriety. A few of the pics seemed to have come out of nowhere, though, and rose through the ranks because the imagery or the story behind the photo are so compelling.

A few caveats about the list: Skylines just took into account photos from the English-speaking world. The ranking you see is also a snapshot of the time period. If you click through to the original, the tweet counts will invariably differ from the period in which Skylines made its determination.