The official Twitter for iOS has been updated to version 5.12 which Twitter for Android has been updated to version 4.1.9. The update brings a few new features and improvements including inline previews for photos and videos. The feature, enabled by default but toggleable in the settings menu, shows an expanded preview of photos and Vines (but not YouTube videos) directly in your feed. You can then tap the inline image to show a larger version.
You can download the update directly on your device through the respective App Stores or by using the following links:
Additionally the update also brings reply, retweet and favorite buttons directly underneath each tweet, meaning you no longer have to long press to access these. (However, you'll still need to long press to share individual tweets to other apps.)