Go into the user CP top left underneath google search
from the left hand menu click change profile picture
If the picture you want is on your computer choose option 2 use the browse button and find the picture you want from your hard drive
If the picture is online choose option 1 and put the web address of the picture in the box
The only real problem you might have is if the picture is too big you might need to resize it,
URL stands for uniform resource locator

it is basically the web address an example of this would be right click on your AV pic scroll down to properties it will give you the address of your AV
I hope that helps
JC