Scaling Images proportionally

I have read hundreds of pages of forums to try to get my images to scale proportionally. If I get the thumbnail to look proportionate, the large image in the post is stretched horribly. If I get the large image to look good, the thumbnail is squeezed.

Here’s my webpage: https://midwestwindowbroker.com/wordpress/category/roof/

If you take a look at the Asphalt or Steel shingle posts, they look good. The Steel panel on the other hand is stretched vertically. If you click on each respectively the Asphalt or Steel shingle pictures in the posts are now short and fat and the Steel panel is correct. I have tried several plugins, I have tried to find the codes that were posted in other forums tried changing the width of the picture. So far nothing has helped.