Images/thumbnails cropping rather than resizing

Hi

I’m not sure if I’m missing something, but I am wanting to setup a [non interactive] gallery displaying client logos. I would like the images/thumbnails to be constrained to maximum height and width, but I don’t want them cropped!! At the moment they are all cropped and you can’t see the whole logo.

What am I missing? I can’t for the life of me see how I can change this.

Thanks