Images squeezed, not cropped

My theme has dropped Tim Thumb for image scaling/cropping and now uses the built-in image handling of WordPress instead. I have hundreds of images associated with posts over many years. But whereas Tim Thumb would automatically scale and crop them to fit different requirements (e.g. the slider or thumbnail sizes on the home page and index pages), the new built-in WordPress image handler just squeezes them to fill the shape. For glaring examples in the slider, I can manually re-edit the featured image so that it is of the same proportion as the slider’s size. But do I just have to learn to live with the distorted thumbnails or will WordPress come up with a solution? My website is: https://skymania.com/wp

Thank you,

Paul