For your own picture: You go to your profile, then click on the picture where it says "change". There will be two options, either upload image from gallery or upload your own photo. Click the button that says browse. You will then get to pick from your personal photos. After you find your picture, click "upload", then "save changes"
For one from the gallery: Click on yor profile, then your picture, then browse through the gallery, and hit save changes