I suspect it might be to do with adding them into My Uploads instead of into Site Assets, from the wysiwyg.

Adding them into Site Assets is generally best for images/files you want to be publicly visible.

(aside: I hadn't actually realised that My Uploads was visible to all users as i don't see it when I log in as sitewide admin. I will have a think about how to make this better and more accessible to all)

In the meantime, I definitely recommend using the Site Assets. You can add them directly into here from the Insert/Edit image dialogue in the editor, or by going to Assets Library in the admin pages.

Thanks

Joe