When you post a comment or discussion, click on the following icon:

The window that appears is called "Media Manager" :

In "Shared Media" you will find all the resources shared with all forum users by the moderation team. You can use the elements but cannot delete them.
In "My Media" you will find all the files you have previously uploaded and can reuse.
Note : If you delete a file/image in your "Media Manager", it will also be deleted from all comments/discussions where you have used it.
Click on "Upload" at the top right of the screen to upload a file/image, you can select several at the same time :

Once the file(s) is/are uploaded it is automatically selected, you just have to click on "Select file"/"Select files".
The window will disappear and will then display something like this above in your discussion/comment editor at the cursor position :

This string is the link to the media file that will automatically be displayed in your discussion/comment when posted.
⚠ Do not change or alter this text.