Go to the drop down menu on the top right of every page and select Account settings
Click on the Upload new image link that appears
Using the explorer/finder window that appears, select your photos and click on the Open button
The square area for cropping will appear within the picture, move it around until you like what you see, then select Crop & Save
Note: You can adjust the crop position and size (small or big), but you cannot change its aspect ratio. For changing aspect, you should crop/adjust in an image editing software (e.g. Photoshop or MS Paint) and then upload again.
Your original (un-cropped) picture will be stored in case you change your mind and want to crop it differently. You can change the selection of cropping any time you wish.