Ricky commented on Add border around product images

It looks like your images aren't perfect squares. So part of the white background is showing through.

If you change this to

.four-per-row .coll-image-wrap, .three-per-row .coll-image-wrap {
  overflow:hidden;
  text-align:center;
  position:relative;
  background-color:#ffffff;
}

to this (for demonstration purposes only)

.four-per-row .coll-image-wrap, .three-per-row .coll-image-wrap {
  overflow:hidden;
  text-align:center;
  position:relative;
  background-color: red;
}

you will clearly see the background.

If you can make the images square, it would probably be the easiest fix.