Tagging separate files
In both Drupal 6 and Drupal 7 files are not used independently. Most often, fields are used through a file field, on either a node (D6 or D&amp;) or an entity (D7).

To do the above, I would suggest using a file field on something, that holds all the info about the file. You could add things like title, teaser  text, description in addition to tagging. For Drupal 6 the only option you have is to use nodes, for Drupal 7 you could use nodes or create your own entity in a custom module. The easiest thing will be to use nodes, as you wont need to create a lot of functions for editing, loading, rending... the entity. Nodes are probably also a good match in this case since the files are content on your site.

Another plus doing this, is that you would be able to use the Views module, and create different galleries or list of image files.