By default, your avatar starts off as the first letter of your username. To upload a picture to replace that, click on your avatar in the upper right, then click the gear icon:
Once on your profile page, click on the pencil icon next to your Profile Picture and upload your own image.
You don’t have to use a photo of yourself if you’re shy - a picture of a pet, a flower or an emoji is totally fine.
Thanks for the heads up @Nic