In hubsell, it is only possible to add images to your signature from a live URL. So, here is how to do it.
Source your desired image from a live URL in a separate tab.
Go to settings > email signature.
Scroll down to the signature section, click on the Insert image icon and upload your image
Adjust the size of your photo from the corner of the photo.
Optionally you can hyperlink your image to any preferred destination e.g.: your homepage
Now you know how to add an image to your signature. Do not forget to click on Save changes when you are done.