Once you've taken or uploaded a photo, you can add music before sharing it.

Instagram app for iPhone
- Tap
at the top, then scroll to Post at the bottom. - Take a photo or choose one from your phone's library.
- Tap Next, then tap Next again in the top right.
- Tap Add music. You can select a song from the list or search for a song in our music library.
- Use the sliding bar at the bottom to choose which part of the song you want to play with your photo.
- Tap Done, then tap Share in the top right.