The default image on the TARS widget button looks like this -
You can pick another bot image from 10 different options like this -
Or you can upload a custom image like this:
Click on the chatbot widget and then you would see an icon to upload a custom image.
Clicking on this upload image icon would allow you to upload any custom image from your device.
Once you upload the image, hit save and that's it. You do not need to change the code snippet on your website after making this change.