If this is a public forum, you will want to think twice about allowing people to upload files directly to your server/website. Using a simple dialog and making them put their image on another service is ideal and much more secure.
If you must enable this, in v8 I believe you can go to /CMSAdminControls/CKEditor/config.js and add 'QuicklyInsertImage'
to the config.toolbar_Forum definition.