Sorry, lost track of this and just noticed your marketplace post
Does this work for you when added to /head?
<script type="text/discourse-plugin" version="0.8">
const TopicRoute = require("discourse/routes/topic").default;
TopicRoute.reopen({
activate: function() {
this._super();
Em.run.next(function() {
$('body').on('contextmenu', '.cooked img, .mfp-img', function(e){ return false; });
});
}
});
</script>