Discussion about a Story can take place in the Comments & Attachments section of the Story dialog (while discussion about Epics can take place in the Epic Discussions tab).
You can reference or call attention to a Team Member by typing "@" and then typing or selecting the person's username in the dropdown.
Adding Images to Comments
You can add images to the comments field in three ways:
1) select the image icon to upload into the comment
2) drag and drop the image from your desktop into the comment area
3) copy/paste the image from your clipboard (NOTE: Copy/paste is currently only supported in Chrome. It will be added to other browsers when it becomes available in the spec).
Animated GIFs will become static GIFS when copy pasted.
Linking to comments or files
If you want to reference a comment or file from another story (or even the same story), you can get the comment's unique URL by hovering over the comment, right clicking and copying the link location.