I don't know of a plugin that
will do that for you. This
might -
condition (if you can extend it to test for tags in the content).
Take a look at
event handlers, you want to add DokuWiki code if a tag is found in the content (data field). See also
action plugins.
It
might be possible to do this with CSS, but it seems complicated, may not match your use case (you don't say where in the page you wish to display images, or how tags are created) - and
I have not given the subject much thought. However I am very interested in your idea and will watch this thread in the hope that others have some useful input.
If you have root access then it would be possible to setup a cron job that greps for tags and inserts the appropriate image into the page, from an array, at the right place. This may not suit your use case, especially if you want an instant result (though basing it on a file change monitor would).