[possible bug] image rollover isn't covering whole image

Hi guys – I think I’ve noticed a bug.
I’ve noticed that when I put my images in the 3 or 4 column layout – the opacity rollover doesn’t reach the end of the image.
This only seems to happen in the middle images of either of this layout.
So for the 3 column layout – the second image rollover doesn’t cover the image properly and for the 4 column layout, the second and third rollover doesn’t cover the image properly. I can’t take a screenshot so have included login details below for you to see. All of the images are the same dimensions so it can’t be this. I had a peek at the CSS but can’t work out what’s causing it.

Here is an example page with the bug – please roll your mouse over the second and third image to see what I’m talking about. I’m using the latest Chrome on Mac but my client picked this up while viewing in IE.

http://www.acmena.com.au/services/

Any clues as to how to fix this?
Thanks