feature is implemented only for Generic Content based modules, but Forums module is not based on this concept, therefore using Tags or Metafields is not possible for time being.
Ordering images from Libraries is completely possible. You just need to add new Metafield like described in the User Manual -> Working with Modules -> Images and Documents -> Customizing Images and Documents
, section Add custom meta fields
. Please keep in mind the information from the KB article How to apply the new metafields to old items in a module
when you add new meta fields for your existing items.
As additional information on these issues, we plan to change the Forums module in future. We will probably convert this module to a Generic Content based one, but this will not happen before the end of this year.
As you mentioned feature requests, we will greatly appreciate if you elaborate what exactly you want to see in this module.
the Telerik team