Hi Frank,
You have a variety of logos which are all different dimensions.
There are a few ways to over come this.
Use CSS to align them.
Edit your logo images so that they all have the same overall dimensions.
Create one image with your logos pre aligned, but then you would only have one instance of img_greyscale.
I opted to use an image sprite, all logos in one image, then created individual DIVs per logo and use a combination of position:relative and background-position to align the sprite to just show the image required.
I had no luck getting the img_greyscale to work, but as I had my own greyscale images of my colour logos, I just created an additional on hover class that called up the greyscale image sprite, with the same alignment positions.
Hope that gives some ideas.
Simon