It's a pretty common issue, if not as much as it was in the early days of the Web. The problem is that while modern monitors run at 2.2 gamma, including Apple monitors, the Mac UI is still built for 1.8 like back in the day, so if you run your Mac at the settings everybody else is using, the UI looks too dark.

Anyway, this is probably more like what you wanted to show, although since it was already compressed, changing the bright levels around highlights the artifacts. I'm sure the publisher has web graphics ready or can provide them at the proper gamma - any Mac-based web designer knows they have to compensate for all the other machines in the world.