Unfortunately, adding a photo as an attachment to the calendar still doesn’t work.
Hi there and welcome to the Community, @Pawel_Stana_Stankiew 
Unfortunately, there is an issue with images pulled through Google Calendar. It may appear randomly for a small part of the users, and our dev team is already on it.
As a workaround, you could try to add a link to the image to the event description, and it will make it appear in the widget.
The image needs to be publicly accessible via a link (e.g., uploaded to an image hosting platform (like Imgur).
Please try it out and let me know if it worked 