Merge branch 'docs-image-discussion' into 'master'

Add image discussion docs

Closes #38879

See merge request gitlab-org/gitlab-ce!14743

doc/user/discussions/index.md

@@ -153,6 +153,43 @@ comments in greater detail.
 
 ![Discussion comment](img/discussion_comment.png)
 
+## Image discussions
+
+> [Introduced][ce-14531] in GitLab 10.1.
+
+Sometimes a discussion is revolved around an image. With image discussions,
+you can easily target a specific coordinate of an image and start a discussion
+around it. Image discussions are available in merge requests and commit detail views.
+
+To start an image discussion, hover your mouse over the image. Your mouse pointer
+should convert into an icon, indicating that the image is available for commenting.
+Simply click anywhere on the image to create a new discussion.
+
+![Start image discussion](img/start_image_discussion.gif)
+
+After you click on the image, a comment form will be displayed that would be the start
+of your discussion. Once you save your comment, you will see a new badge displayed on
+top of your image. This badge represents your discussion.
+
+>**Note:**
+This discussion badge is typically associated with a number that is only used as a visual
+reference for each discussion. In the merge request discussion tab,
+this badge will be indicated with a comment icon since each discussion will render a new
+image section.
+
+Image discussions also work on diffs that replace an existing image. In this diff view
+mode, you can toggle the different view modes and still see the discussion point badges.
+
+| 2-up | Swipe | Onion Skin |
+| :-----------: | :----------: | :----------: |
+| ![2-up view](img/two_up_view.png) | ![swipe view](img/swipe_view.png) | ![onion skin view](img/onion_skin_view.png) |
+
+Image discussions also work well with resolvable discussions. Resolved discussions
+on diffs (not on the merge request discussion tab) will appear collapsed on page
+load and will have a corresponding badge counter to match the counter on the image.
+
+![Image resolved discussion](img/image_resolved_discussion.png)
+
 ## Lock discussions
 
 > [Introduced][ce-14531] in GitLab 10.1.