You can add your own images to comments using 'Attach a file' below the comment.
By default this adds your image as a thumbnail, which can be clicked to enlarge. To add the image into the comment itself, hover the cursor over this thumbnail and select 'Insert image'. Large images will be re-sized to fit the width of the message window.
If you delete an image from a message that has been posted, you will not be able to add any more. In this case, copy the text from your first message into a new message and use 'Attach a file' again. You can then delete the first message.
Messages can only be edited for one hour after they are first posted.