For guest users, we utilize various properties provided by your computer/tablet/phone to identify you as a unique individual. But, this method is not 100% unique and sometimes a prior guest has the exact same hardware identity as you.
If a prior user with your exact hardware identity properties has previously uploaded a photo, it will shown when you enter chat, regardless of the guest name you use.
You have two options:
Upload a new photo to replace the one currently showing.
Create a free, registered account and the photo will always be what you have chosen (or a generic avatar if you never upload a photo)