After logging in you'll be taken to your Crush Page. A pencil icon will be to the right of the profile picture.
Click the pencil icon to be taken to the upload and cropping page.
Upload a picture from your device and crop it if necessary. The slider on the right will zoom in or out of your image. A live, actual size, preview will show up at the bottom.
Once you're happy with the way it looks, click "Save Picture" at the bottom of the page.