Here:
Go into profile at the top of the page. Select edit in the top right. Select "Avatar Image" at the top. If you have an image to upload as your picture, select Upload Image and navigate to the picture and select it. Press open. If you want to use a stock one, select the bubbles next to the default one you would like to use. Once this is done, select "Keep Current Avatar" at the top of the window. Press "Save" and your profile picture should be saved!