There are two filters
wfea_cal_list_extra_js to help with this.
This is a more complex option and is you need support please do raise a support ticket.
The following is an example that adds the image to the calendar front end – not just the tool tip. It has been implemented as a mini plugin which you can use as-is or modify to your needs.
If you want to use as-is you can get the zip file from here: