I didn't think about that situation. Here's what's happening... In earlier versions, the box-shadow on the slide image was using true black - rgb(0, 0, 0). Even when only partially opaque, that would be invisible on a black background. But to soften up the shadows, I changed to a dark gray - rgb (51, 51, 51). That, of course, is lighter than the black background, so it shows up as a "glow."
The fix is simple enough - turn off the shadows on your slide images. They must have been invisible before Gromit 57, anyway. I see they're already off for the thumbnails.
Either that, or do some custom CSS and create something really interesting - see screenshot.