The images thumbs are now created on demand, a much better system than “old” Connections where the image settings needed to be set ahead of time. Simple changing the settings should create the new images when there need to be displayed on the page. The file names are changed to a “hash” of the defined settings to ensure the browser cached image are not used; ie the old versions of the images.
With that said, there are templates which enforce a size, Slim both version and Except both versions. Those have always defaulted to the Crop and resize proportionally to best fit the specified dimensions, maintaining the aspect ratio. option with a hard coded size.
Are you using one of these templates?
Can you share a link to the page?
Can you attach a screenshot of the images settings?