Step by step guide:
1) Click on "User CP"
2) Click on "Edit Avatar"
You have two choices now. If the picture you want is on your computer simply click the "browse button" located in the "edit avatar" section, find the picture you want and you will have it.
If the picture you want is online simply go to the picture, right click and select "Copy image location" paste the link in the "Enter URL to Image" section and save it.