I haven't found anyway to get those to work without the following steps:
1. Write your post, and add the attachment to it. But just leave the attachments as the default "End-of-post expandable thumbnail". Don't try and put them inline yet.
2. Finish the post, then immediately click "modify" to start editing it again.
3. Expand the "Attachments and other options" underneath the text box, it should list the existing attachments.
4. Place your text cursor in the post where you want the inline thumbnail to appear.
5. Click "Insert Attachment x in this message" next to the attachment you want to add inline.
6. A "attach=x" label should appear in the text box.
7. Save the post. Now it should display inline attachments correctly.
I just post the message, then right click on the preview image in the post and select 'copy link address'
(Since clicking the preview would open the full size image copying the link address gets you a url to the full image. Don't select 'copy image address' or you will get a url to the small preview image. Unless the image is already large, then the logic is reversed)
Then I edit my message, put the cursor where i want an image, click the image button, Ctrl-V the url, and click the image button again. That gets you the img tag - url - img tag
Then save the message.
Done
It's fast for doing 1 image inline, but to do multiple inline in a single post it's easier to duplicate the browser tab before editing, that way you have a tab you can copy each link address from. Or you need a clipboard history.