Anyone can currently add notes to images from Commons, and changes to them are patrolled for malicious content just like any text you might add to Wikipedia. If you're familiar with adding notes to Flickr or any other photo sharing site, then this should be a no brainer for you.

Once you find the image you want, either by clicking through from Wikipedia or searching Wikimedia Commons directly, just click the "add note" button underneath any image on its own page. Then type in your text and either preview or save it. In addition to simple text notes, you can include wiki markup to add links or for styling, and if your annotation fails then you can add it manually through editing the page's code.

The image annotation gadget is basically just some JavaScript written by a Wikimedia volunteer and made standard on the site. If you're a regular user and you don't like the boxes showing up on your images, then you can turn it off via your user preferences. The advantage to logging in to add notes is that a history of all your annotations will show up in your user contributions history. According to the project's FAQ, a version will soon be available for use directly within Wikipedia.