What I do to make a thumbnail is to open a new blank image in the dimensions of the thumbnail (200x200 pixels). Then I copy my gallery image and paste it as a new layer to the blank image and then move the image layer around until I see something I like. I then use "File" and "Export" and pick "jpg" and then adjust the compression to the minimum number that squeaks me just under the 15 kb in size limit. I always end up with a good quality thumbnail doing that.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi