It is indeed limited to the main image, because additional images open in a new window as a "raw" image: there is no page around it to add any control.
I could add something to open them in a javascript popup, to have the ability to also display a brighten button. That would also be more handy as you can easily close it (rather than having to go back to the page).
If one wants to open an image in a new tab as currently, it would still work, but without brighten button though.
I've added it to my to-do list, but there are a lot of things on it