How to scale image with equal width and height without distortion?

I have a section on my frontpage that querys through recent posts and grabs the title and thumbnail of each. In my functions.php, I’ve specified a width and height for the thumbnail using:

add_image_size( 'thumbnail-small', 179, 147 );

I have all different size images in my posts set as featured images and when they get pulled into the image thumbnail element on the page, they crop proportionately, but don’t scale to the correct size:

I put borders around the images so you can see the scaling, but it seems as WordPress is just scaling them to whatever fits right.

How can I get them to all look the same, and then just crop out the excess with overflow:hidden in CSS?

I’ve tried some CSS such as:

// .image-mask is the wrapping div
#middle .image-mask { width:179px; height:147px; overflow: hidden; position:relative; }
#middle img { position:absolute; display: block; top:0; left:0; }

and…

#middle img{ width: auto; max-width: 150px; height: auto; max-height: 100px; }