The following instructions step through the process of changing your Office 365 Account photo within the Outlook Web App.
Click the account image icon in the upper right corner of the window and select My Profile.
On the account profile page, click the camera icon next to your current profile picture.
A window will pop up allowing you to edit or upload a photo. Click Upload a new photo, and browse your local hard drive to upload a profile picture.
Once you've selected a new photo, click - or + to change the zoom, and drag the photo to adjust what shows in the window as needed. Click Set as profile photo to update your new account picture.
The blue check mark will confirm the change, and your account photo will be updated.