How to Scale Images without screwing it up

I used GTmetrix to measure my site speed, and they said I need to scale my images. I went to my media library, choose a product image, and tried to scale it. All my product images are uploaded at 720 X 720. According to GTmetrix, I need to scale them down to 197×197. Here is the problem. My products are listed on my website as thumbnails so where when you click on them, it takes you to a larger picture. When I scale the image down, it makes the the larger image smaller, so when you click on the thumbnail the other image is now the same size, if not smaller than the thumb nail. What the?

How do I fix this??

Please any help will much appreciated!!

Thanks!!