Markdown Editing Help Chat uses a limited subset. You can't use formatting in multi-line messages (except for making all fixed-width), headers and everything that would only be useful in multi-line messages. Images can't be inlined, you can only post them in separate messages, like this:
!http://path/to/ima.ge
For tags, you can use
[tag:tagname]
and
[meta-tag:tagname]
.