Here is a cool trick if your looking for a way to make a photo dark by default and then have it lighten up when a person moves mouse over. I chose 70% as default brightness because it isn’t too dark and it isn’t too bright. Some other CSS styles I have tested don’t work in getting the brightness effect, but this one works fine.
If you want to see a demo of this look to the Posts grid on top right of this article. The thumbnails on that Grid and the thumbnails on the Grid below my author Bio both lighten up on hover, and they are at 70% brightness by default. Go ahead and give that image light!