So if you are uploading photos directly to the thread, the photos will only display in the thread in that specific comment, any other way of linking uploaded photos will just display a link to the attachment stored in that other thread.
If you click the "media" tab at the top on the main menu, there are options to upload media and you can link those to multiple threads if I remember correctly, I don't ever use that function.
The best way to store and link photos to multiple threads is, unfortunately, 3rd-party image hosting websites.