How do you rate someones photo?

Hi I'm pretty new to this new website, I've been looking at some of the excellent users photos and I'd love to rate the ones I like but I cant seem to. I assume the stars are supposed to highlight when you hover the curser over them but no amount of clicking on the stars or the "ratings" section at the bottom of the photo does anything. Am I being thick? lol .

If you write a comment in the users gallery below the picture - At the bottom you will see a row of faint stars to click your rating