Welcome to BYC!
That is no problem.
Steps:
Step 1: Go to your profile, it is on the Top Right Corner of the website page. It says "My Profile"
Step 2: Under your name and your current pic, there should be button saying Replace Avatar, or Add Avatar. Click that.
Step 3: Then a box should open saying "ADD OR UPLOAD AN IMAGE" and then you can choose a file and click submit! All done!
Hope his helps, and I hope you enjoy all the benefits of BackyardChickens.com!