I listend for 12 hours on the streaming API statuses/sample stream
https://dev.twitter.com/docs/api/1/get/statuses/sample
During several samples, it turn out that about 5% of the tweets contain pictures from the twitter picture services.
Since random is never random, the question is if this 5% are influenced by the random selection, or if really 5% of the tweets contain pictures from twitters image server.