There’s a way to post pics - but it's far from intuitive. Post the reply, then in the web interface, click your picture, pick stories, pick replies, click the 3 dots, pick edit. Now you're in the main editor (like for writing stories). I mostly use it for embedding links, as in the mobile app, readers can't click on an in-text link - and they also can't copy text, so there's no way to 'get' to that link without typing it in manually. But inserting pics works too.

[edit] you may be thinking: “But what about anyone who reads/responds after I’ve hit reply but before I’ve edited?” Yeah, that’s bitten me, too. Now I usually put ‘EDITING’ in caps at the bottom when I reply (but intend to edit it) and make a point of getting to the editing with minimal delay. [/edit]