1. Press the Profile option at the top of the screen.
2. Scroll down to the bottom of the screen, you should see the image below

Your image will have to fix the requirements listed. One one to edit it is use Microsoft Paint - you can resize the image and save it so that it fits the size limitations.
If you have the avatar on your computer - use the browse button. If you are getting one from the Internet, then paste in the URL of the location (most people will use the first option).
You could always Google "Avatars" and find that there is a whole range of different avatars out there.