Hey there,
If you need it on specific posts, then follow @Lely’s suggestion. Open your post in WP post editor, under Post Settings, find Body CSS Class(es) and enter a custom class for example no-margin and then go to Appearance > Customize > Custom > Edit Global CSS: and paste the following code:
.no-margin img {
margin-bottom: 0 !important;
}

This will be applied to that post only. And you can reuse this code for any of your posts by just adding no-margin class to the posts’s Body CSS Class(es) field.
Hope this helps!