According to the photo search engine Skylines, iOS devices account for 39% of all photos posted on Twitter. Of the 24 million photos posted to Twitter in the week of October 22nd to October 29th, Twitter for iPhone came in highest with 21%, Instagram accounted for 13% and the iOS 5 Camera and Photos app accounted for 5%, bringing the total to 39%.
Apple's new iOS 5 introduced integrated Twitter functionality, bringing posting of photos to Twitter directly from the Camera and Photos App. iOS 5 has only been out since October 12th and it's already the 7th biggest photo client for Twitter.
Other big players were Twitter for Blackberry at 12% and Twitter for Android at 10%. Twitpic accounted for 8%, but was not counted towards the iOS percentage because it is a multiplatform client.
Read Skylines' report on their blog here.