Crop (Not Resize) Images

Hello,

I have site theme “fullscreen” on website https://www.conceptualcanvas.com. When I insert an image into a post, it automatically generates a thumbnail image for the homepage (see website).

I can’t find any information on how to force it to crop a specific area and not resize.

To give you an example of what I want it to do, look at the John Lennon pictures. That image has been scaled down and then cropped to create the image you see on the home page. I would like it to just show a section of the image without resizing. The point is to make it difficult (and intriguing) to see what it is.

Can anyone help me out please? Thanks.