To post photos and videos, tap on the plus button in the center of the tab bar at the bottom of your screen. The first thing you will see is the most recent photo and videos from your camera roll. Tap on any that you would like to share – you will see a blue number appear over the photos you've selected.
Once you are ready to post, click on the blue arrow near the bottom right of your screen. The composer screen will dismiss, and you will see the content you selected appear in the main feed along with a upload progress indicator. It is recommended that you wait until the upload completes before leaving the app.
If you do leave the app before the upload completes, and for some reason it is unsuccessful, you will get a notification that the upload failed and you can return to the app to retry it.