On a lot of the image boards described by OP, tagging is managed by the users collectively. That is, almost everyone could not only add but also remove tags from content, as well as collectively maintain wiki on what the individual tags mean. When multiple similar tags meaning same thing come up, they’d alias to one central one; when different usages of same tag come up, they’d take a Wikipedia-esque approach to differentiate them; some even go as far as creating categories for tags so similar concepts can be grouped together. Trouble makers (people who repeatedly use tags incorrectly) lose their tagging privileges and so problem is kind of managed at bay.