MOBILE: Go to your main screen (the one with all your spaces listed) and click the "Settings" icon on the blue task bar at the top of the screen (next to where it says "Your Spaces"). Tap "Edit your Profile". Tap on your current photo and then you can either take a new photo or select from pictures on your device.
WEB: In the "Your Spaces" panel (where it lists all of your spaces) click the "Settings" gear wheel icon in the top left-hand corner. Click "Edit Profile" from the drop-down. Click on your current photo and you can upload a new profile picture from your computer.