If you chose to add a photo, you will have the option of editing your photo once it has finished uploading.

The slidebar to the right lets you zoom in and out and the red-bordered crop-box lets you select the portion of you photo that you would like to use.

When you are satisfied with your selection, select the Crop button.

After editing your photo, the result will be displayed to you as it will appear to others.

If you like what you see, select the This picture looks fine button, otherwise select the Discard this picture, I'll do this later button.

Remember, you can change your photo later so it doesn't matter too much if it doesn't look perfect.