Slight photo resizing on mouse - could be CSS related

http://traffic.mg/

Offending browser is: Chrome OSX
Safari is fine

A site I'm working on is giving me fits with a photo mouse over slight resize issue:

It's very slight, and only on on the center column photos. You will notice a more or less 2 pixel shift on hover when the image transitions from greyscale to color.

There is some tricky math on the CSS for the these columns and it's beyond me how to troubleshoot this.

Any thoughts on the solve on this one?