blog page image vs thumbnail image

I would like my blog posting featured image to be one size image and the thumbnail image used on the Classic Small template be a smaller version of the same image.

The image editing capability is not only cryptic at best, but it only allows me to change the featured image which might make the thumbnail look ok, but then the featured large image looks terrible.

If you are going to direct me to CSS code, please advise me of what specific code I should alter. I don’t have time to go fishing through navigation for file names.

Your product is NOT delivering on the capabilities advertised…

Thanks