Image tagger

I'm looking for an image tagger... been looking for several hours, with no luck; hopefully someone can help me out.

I need the image tagger to have the following:

* Must be completely portable
* Must be able to put multiple tags on an image
* Must be able to handle images from any folder
* Must be able to search for an image based on its tag(s)

(If anybody knows of a general media tagger - handles audio, video, graphics, etc. - that can meet my above criteria, that would be even better...)

TIA!