How can you post an image in the chat rooms instead of providing its link?
There should be a "How to Format" within the chat rooms as well.
There's no difference; if you post a link on a line by itself (it must be by itself, however) it's automatically oneboxed. In the case of image links, that means displaying the image instead of the link. If you want to upload an image instead, there's an "upload..." button next to the text field you can use:
In most cases, it will automatically detect an image from the url (based on the ending); if it is unclear, just add a leading
! and it will try extra hard to think of it as an image.