If you click on an image in Wikipedia, it will try to open up its File page. Most File pages reside in Commons. Without the Commons wiki, you will not get more metadata on the image, nor will you be able to download the full size version
For example, http://en.wikipedia.org/wiki/Earth has an image called File:Solar_Life_Cycle.svg. If you have Commons and click on the image, you will get a view that is similar to http://commons.wikimedia.org/wiki/File:Solar_Life_Cycle.svg
Note that Wikipedia may display images under an "en.wikipedia.org" url, but it has actually retrieved the data from commons.wikimedia.org.
If you click on the main image on its File Page, it will download the original size image and open it in your image viewer. For example, clicking on the image at http://commons.wikimedia.org/wiki/File:Mona_Lisa,_by_Leonardo_da_Vinci,_from_C2RMF_retouched.jpg will download and open the full 7,479 × 11,146 image (89.9 MB).