thumbnails are small versions of an image, that contain the entire image, there no cropping on the web site.
A possible solution is to create two images on your computer with software the "actual" and the "cropped thumbnail." and upload them both. you can remove or move your "actual image" to another page.
When you upload your cropped images you can link them to your actual images in the Wp image uploader.
The Downside to doing this, over just using the thumbnail generated WPs uploader is: that your "Actual Image" may lose some Search engine power by not being connected to the post or page.
The other downside is it takes more time and work to do this, until you get the hang of it.
----just giving an option how to resolve is, along with the option of just using the shrunken image generated by the uploader.