Hi, not sure if you're still having this issue, but I do something different which may work:
If you build your .ics file corrently and attach it in a mail (even a test mail is fine). Send yourself the email and when you click the .ics file, make sure you save this file to your local desktop. You'll have this file stored locally now, then upload this file into the desgin studio (I have a folder for .ics files). Once uploaded you'll have the marketo url for the file. You should now be able to use this in your email/image.
Hope that helps.