I'm a UX newbie and I'm trying to design a page of image thumbnails. I came up with this:

The trouble is that this layout creates the Hermann Grid illusion. When looking at the grid, most people see flickering black dots in the corners. These dots don't exist in reality; they're created by the human visual system. They're nonetheless very distracting.
Can anyone suggest how I might modify my layout to avoid creating this optical illusion?