1. Go to your profile page by clicking "My Profile" in the menu bar:
2. Click on any of a) your current profile picture; b) the "Photo" link below it; or c) the "Settings" link near upper right:
3. Click on the button to select a photo from your computer. The button caption will vary between operating systems and browsers; typically "Choose" or "Browse":
After selecting an image file from your computer, scroll all the way to the bottom
of the page and click "Save":
Not that due to browser caching, it may take a few minutes for the change to become visible.