Unfortunately, attachments won’t show up in calendar or gantt views as there is no option for it in those views. There is an option to show them in timeline, however, it will only display the image text of the file, not the image itself.
I have worked around this for a client by creating a gallery view and sorting by date, which they used in conjunction with a calendar. This may not be ideal for your situation.
Are you asking for the option to add attachments to a calendar or gantt view or the structure I built for another client?
If its what I’ve created, it was pretty straight forward. I set up a gallery view where attachments were visible. Then, I had a date field, which was used to note the first of every month. Then, to label each record, I had a formula field that reformatted the date field with just month and year. That way, each icon showed the month and the image. It was for creating a calendar as well.
I’d be happy to walk you through it in more detail or even help you build it, if that is what you are looking for.