please elaborate on "in the past." because you can still do this. no feature was removed or added to prevent doing this.
if a forum is setup with the stock post attachment features enabled, you dont need to post your attachments with any third-party website or software.
lightbox (you referred to it "shadow box") is a photo gallery presentation option. there is nothing stopping you from getting the url to the full sized attached image by right-clicking (or long-pressing if on mobile) on the attachment preview thumbnail, and choosing "copy image url" to use that inlined anywhere within your post.
