tu go to your perfil and click on photos. Once there tu can subir the foto tu want. Then, below the uploaded image, there are three icons. One of which is a teeny person which is make the image your perfil picture. Click it and that foto will become your perfil picture. tu can also make it your perfil picture by, as your uploading, clicking the "make this image my perfil picture" box and then it will change your perfil picture once the foto is uploaded.
