The iOS 5 Photo and Camera app now represent the 7th biggest Twitter client for posting photos, with a 5% share of the market, according to numbers gathered by photo search engine Skylines.

iOS 5 follows up Twitpic, Twitter for Android, Twitter for BlackBerry, Instagram, the Web and Twitter for iPhone rounding up the top of the pack.

Combined, the iPhone accounts for at least 39% of all photos posted to Twitter, largely due to Instagram and Twitter for iPhone. There are a few percent in the smaller categories like Echofon, Others and Tweetdeck that aren’t able to be attributed solely to iOS, as they are multiplatform.

twitter clients used to share photos1 1 520x416 iOS 5 now 7th biggest Twitter photo client and 39% of all posted are on iPhone

With the integration of Twitter into iOS 5, not only did Twitter score themselves a deep toehold on one of the best mobile operating systems around, but it also nearly guaranteed itself longevity.

iOS 5 has been adopted quickly by users of the iPhone, with recent numbers putting it at roughly 1/3 of all iOS users already upgraded. The massive sales of the iPhone 4S haven’t hurt either, with millions of new users of Twitter being minted by the month. Twitter has been pushing the integration hard too, with a fancy landing page aimed at demonstrating how Twitter integrates with iOS.

Twitter also moved to its own hosting service for posting photos from its web app, official apps, and of course, iOS 5. This means that Twitter now leads the percentage of images hosted on Twitter, with 36.1% versus 30.3% for the previous leader TwitPic.

This is nice growth for Twitter, but I have a feeling that it is just the beginning. Apple is going to sell huge numbers of iPhone and iPad over the Christmas season and every one of them will have an invitation to install the Twitter app waiting for them in its Settings.

Yes, having the integration there doesn’t guarantee a new Twitter user, but CEO Dick Costolo has already said that Twitter has seen a 3x jump in signups since the launch of iOS 5. This is just the beginning. Look for iOS 5 to become one of the top 3 Twitter clients for posting photos by the beginning of next year.