How to add a category to a MediaWiki Page

If you want it from FTP upload it in the main folder and for the image type

<img src="image_name.jpg>

Yes, if you have a list of pages that you want to add to some category, AutoWikiBrowser can help you do that. But, as with any other mass change, you should have a consensus to do so, or the change should be uncontroversial.

Also, to be able to use AWB on Commons, you need to be approved first, see Commons:AutoWikiBrowser for more information.