When you add images, most of the times you would want to describe a little more about the image itself. That's where adding a caption helps you. This would work for images as well as videos. After you add an image/video, tap on the media in the compose screen.
The dialog which let's you specify the size and the alignment, now has a caption field as well.
This little feature would be a part of the next update.
Do let us know what more features you would like us to roll out.
Posted via Blogaway