[eluser]jegbagus[/eluser]
i think you should add 2 more table,
master tag that hold all kind of tag, and tag that connect between the object with the tag.
table :
mstag (tag_id, tag_name, tag_desc, insert_date)
tag (tag_id, obj_id, type)
type is kind of the tagged object (image / post / etc)
obj_id will be the id of tagged object (imageid, postid, etc)