hey MAN, since version 1.8.0 of lay theme you can use brightness instead of background color for the hover. This way you won't get these borders. But yea your image will just change its brightness on mouseover and not its color. I hope that works for you.
Copied from http://laytheme.com/version-history.html
Customizer: "Project Thumbnails" → "Project Thumbnail Mouseover" → new settings for brightness on mouseover. Brightness works better than the "show Background Color" setting if all you need is a darker or brighter thumbnail on mouseover. With brightness you will not experience glitchy 1 pixel lines in Safari on mouseover.