You can not invoke a microflow directly from a mail. A mail is sent to a user that is not logged in to the system and even not reading the mail in a browser. You can add deeplinks to the image and handle the deeplink (maybe anonymously) in your Mendix application. You may also use a online service like rating-widget. (this is just a sample of a service like this, I never used it, disclaimer....)
Edit 1: if you use a deeplink you need a unique id (like guid) to check the deeplink is valid and determine the source. This looks like
https://myapp.mendixcloud.com/link/rating/2050a0dd-9324-4e2b-9438-077e5aae0218