I believe that it’s not currently possible to add an image as a parameter to a triggered email. I suggest that you reach out to Wix Customer Care for assistance with Triggered Emails as they might be aware of a solution.
Wix is constantly working on improving the developer experience. Feel free to make your voice heard. The Wishlist Page is the official platform for requesting new features. You can vote, comment, and track the status of the requested features.
Meanwhile, you can send the email on your own using the techniques outlined in Velo Tutorial: Sending an Email on Form Submission . It’s also worthwhile to take a look at the Sendgrid NPM Integration example.