You’ll now be able to tag up to 10 people in a tweeted photo:

Tagging people in a picture makes conversations around photos fun and easy. And tagging doesn’t affect character count in the Tweet — you can tag up to 10 people in a photo and still have all 140 characters at your disposal, making it easier to connect with your friends. If you’re the one being tagged, you’ll get a notification. You can adjust notifications, as well as who can tag you, in your Settings. To learn more, read this Help Center article.

You can also attach up to 4 photos in a single tweet:

And now, you can also share a series of photos that automatically create a collage. Just tap on a preview to get the full image and slide through the group. The ability to upload multiple photos is starting to roll out today on iPhone, and is coming soon to Android and twitter.com. Whether you’re on iPhone, Android or twitter.com, you can view Tweets with multiple photos.

I’ve tested both of these features on the official Twitter app. They work. Not sure how much work is involved in adding these features in 3rd party apps. Hopefully, the Twitter team gave them some advance notice, so they can roll both features out quickly.