Understanding Fit to Box, Fit to Width etc

I have implemented the File Manipulation > Upload Files behaviour in a CMS I've built but I'm having trouble understanding how best to deal with Portrait and Landscape images.

I have set the Upload Files behaviour to create thumbnails at a specific size (400x300) and I understand that if the image is portrait, or the dimensions aren't 4:3, that the behaviour is filling in a background colour to compensate.

This looks ugly though:
http://www.stranraermusictown.org.uk/gallery-photographs.php?cat=2

Is there a way to simply fill out the thumbnail with the entire image or, at least, make sure it fits width ways? I'm aware there are other settings within the behaviour but I just don't have the time to spend hours trial-and-error'ing all the options.

In the help section it states...
Fit to box - will resize the image to fit within a specified height and width.
Fit width - will resize the image to fit within a specified width.
Fit height - will resize the image to fit within a specified height.
Fixed width and height - will resize all images to a specified width and height while maintaining the width-to-height ratio. If the resized image does not completely match the specified width and height, a fill color is applied.

Fit to box - won't work because portrait images will look squashed.
Fit width - won't work because the height will remain in ratio and therefore all the thumbnails will be of a different size
Fit height - as above but width will remain in ratio
Fixed width and height - leaves ugly spaces around the images

What is the best solution to this?
Thank you
NJ