If you are a Team Owner, you can do it in the settings section. Note: only a Team Owner can do this. If you are not a Team Owner you can always ask him/her to change it!
Steps to change your Team Avatar:
1. In the right-hand corner of the main page, place a mouse on your nickname and click on the “Settings” button.
2. In your settings choose the “Team” section and press the “Update logo” button.
3. Then choose and download the desired from your PC.
4. Next, you need to crop it. Press the “Save changes…” button after cropping it, and “Save changes…” one more time as there two types of picture sizes used for your Team Avatar.
5. Congratulation! Your Team Avatar has been successfully updated.