Step by step
In order to add an image or photo on your avatar.
Tap your avatar icon located at the top left of your screen (when no image is set, the icon display the initials of your first and last name)
Once in your account settings, tap again on your avatar icon
You can chose between choosing a file from your library or take a live selfie
And that's, now everyone all your friends on Sling can easily recognise you!