The onPageChange event should help you:
It’s executed whenever the user navigates in Discourse. $(document).ready isn’t enough because Discourse normally avoids full page reloads.
Something like this could work for you:
<script type="text/discourse-plugin" version="0.8">
api.onPageChange((url, title) => {
$("img").bind("contextmenu",function(e) {
return false;
});
});
</script>
This doesn’t handle posts that are inserted into the page by infinite scrolling, but maybe it’s enough to appease your users 
(I understand this is a #marketplace post and you are looking for a fully baked solution. This isn’t that, but I thought it might still help.)