Click on the circled i located at the top right of your screen.
Especially now that you are reading this step-by-step guide.
After you have set a new emoji, you would be shown the emoji you have set.
All your friends in the chat will see the new photo.
You can also adjust sound and vibration settings.
There, you'll want to turn on Save on capture to automatically save any photos you take in the app to your gallery.
Also, anyone in the thread can change the icon, so if you change it someone could change it right back or to another emoji instead.
Be aware that the video length is limited to 15 seconds.