Twitter will now let you attach up to four images to a Tweet, and tag the people in those photos, all without counting towards your 140-character limit. This is a pretty obvious attempt to beat Instagram and make Twitter your go-to sharing service.
The new features work if you’re using Twitter’s own mobile app. The image picker now lets you add up to four images, and you can add ten tags per Tweet and still have all your characters for the Tweet itself. Anyone you tag will get notified, and when you view a photo full-screen you can swipe through in a kind of mini slideshow. You can also use the setting to decide who can tag you (and you can do this in the settings on the Twitter site so there’s no need to get the app to do it).
Embedded Tweets will also show the new features. The blog post doesn’t mention third-party Twitter apps, but this is one native feature I’d like to have (you can stick your promoted Tweets though, Twitter).
Source: Twitter Blog