Go to the event. Select Attendees. Choose Email.
Adding images to an email requires the image first be uploaded to a web-based service, such as Imgur.
You cannot upload from a local drive.
PLEASE NOTE: It is important the upload link be the direct link to the file. For example, https://imgur.com/u6gXCvD will not work, but https://i.imgur.com/u6gXCvD.png will work.