The reason I would leave it up to the moderators is that they presumably would choose the most representative photo . I might vote for the glamor shots that really aren't representative of a plant, and I'm sure I'm not the only one that would do that.
Besides, I don't imagine that the moderators would have to use the override that often. I do think they should have the option, however. This is not a photography contest, after all. It's a source of information, and the information should be as reliable as possible.