There are differences with the aspect ratio of pictures (normal and enlarged after mouse-over).
In fact, it's when you zoom the picture that you get its normal aspect ratio.
I had 2 possibilities:
1) show the pictures all with the same aspect ratio.
2) show the pictures with their normal aspect ratio.
I've chosen the first solution because the grids looked better, more harmonious.
Also, in your screenshot, you have chosen the style where the picture is in the middle of the cell. I admit that it's a style where the second solution could have worked rather well. Unfortunately, that wouldn't work at all with the other styles.