I originally just did the thumbs as a 60% opacity, but it was a default white background. I hadn't finished off a full gallery, so was showing the guy the Peter Lik page as an example of the effect. Looking at them together, we reckoned the Peter Lik one was more effective, as the reduced opacity on the white background was a bit washed out looking.
So then I was trying different combinations of the brightness, which needed countering with the contrast, and I couldn't quite seem to get it.
But you've hit the mail on the head - reducing the opacity on a black background is the winner!
Many thanks for prepping that example PNG - much appreciated.
Examples help me figure things out. Because of the way the thumbnails load into grey squares on the site, I'd originally thought he had them set over a medium grey. But after I set the canvas black to give the outer thumbnails a border and turned down their opacity, I realized I was about there. I just experimented with the opacity number till it looked right. Without a specific goal, I think most people will go for 5% increments.
Glad I helped!