Whilst the core WP Symposium Pro plugin has a lot of functionality, the forums extensions adds a wealth of additional features.
This extension, adds the option to upload images and documents to forum posts and replies. These are then shown as attachments to those posts/replies, and if clicked on, will be displayed at the original size (or as big as will fit on the screen).
Provided in the Extensions Plugin.
You can use the global shortcode settings via WPS Pro->Shortcodes in the admin dashboard, or add them to the shortcode as options.