I prefer sending mail as a html content, you can refer here on how to do that: https://medium.com/mendix/how-to-send-styled-html-emails-using-email-module-with-templates-and-mxmodel-reflection-mendix-3d80d18019b9
If you want to add Image as attachment you can use the attachment list in the send email activity.
Hope my answer helps. reach me out for any queries.
If its a small image for a signature for example, you can also use a base64 image. Convert your image to base64 image and now you have a string containing your image.
You can use this string in your email template.
Save the string as a default value in a attribute and link that object to your emailtemplate to show it.
Not sure if you are looking for this, but let me know if you did!