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