I feel really embarrassed because I was giving thumbs up to a lot of photo's in the moderator "waiting" queue. I didn't know a thing about many of those plants, I was giving the thumbs up because they were beautiful pictures.
I think the feature has now been removed to make it easier for the moderators. We can always thumbs up pic's after they've been approved. Just click on Plants at the top of the page and those photo's on the left are (I think) the most recent ones that have been approved. If we like what we see we can click to enlarge and then give the thumbs up.