GmailInlineImageFit.user.css

/* ==UserStyle==
@name         Gmail Inline Image Fit
@description  Adapted from https://github.com/igorsantos07/Gmail-Inline-Image-Fit
@author       https://github.com/kibeb
@version      1.0.4
@namespace    https://github.com/kibeb/Styles
@homepageURL  https://github.com/kibeb/Styles
@updateURL    https://kibeb.github.io/Styles/GmailInlineImageFit.user.css
@license      MIT
==/UserStyle== */

@-moz-document domain("mail.google.com") {
  
  .a3s img:not([role=button]):not([role=menu]):not(.ajT) {
    max-width: 100% !important;
    height: auto !important;
  }
}