The default image on the TARS widget button looks like this -
You can pick another Robot image from 10 different options like this - (See How)
But, if you still want to change this to a custom image, you can do that by making one change in the code-snippet that you copied from the Distribute section. Say you have uploaded your custom image on imgur.com at this URL: https://i.imgur.com/u94ng6n.png
near the end of the code snippet, find this part:
and replace it with
Make sure you use your own image URL. That's it. Use this new code-snippet on your site and the bot widget image will be updated to your custom image.
The final code snippet would look something like this -