I have a different problem and i don't think i am the only one on this planet with this issue. When i switch themes in wordpress, some themes display the "thumbnail" of the featured image as the featured image of the post, which cause the little thumbnail to stretch upto 800px (width). It looks horrible! I got its solution as well, i need to re-upload the image and then set it as featured image again. But i can't repeat the process for every post? Kindly, help!