I tried but couldn't get thumbnails like these, in wordpress. Please check this url https://prime.500px.com/galleries/tricks_and_treats
If you notice, there is no cropping. Its just zoomed-out or whatever you call it. Tumblr also uses such technique of thumbnails. The image is shown full but in small size. Width is custom, say 300px and the height of thumbnail is variable, so the thumbnail does not look awkward.
What I did, unchecked the Crop checkbox in Media settings and set dimensions for thumbnail but it wasn't generating the same as on the 500px site.
Any idea or suggestions?