Product Images being distorted despite image sizes bigger than stated

Hi,

For my product images on http://bartonsdrinks.com there are different sizes and then when you click on the images, they are extremely big and I can't work out why it is showing that.

They all need to be the same size despite the image size. I think there is some CSS config that needs to be done, but I can't work out what.

Please see the screenshots attached.