Option to replace individual thumbnails

I forked a branch on Github adding the ability to change individual thumbnails. This is useful if you don’t want your thumbnail to be simply a cropped version of your main image and want to upload a custom image instead.
https://github.com/EricDu/enable-media-replace

Since I’m a novice at programming, I’d appreciate having a few people do additional testing before starting a pull request. I also modified the UI slightly and would appreciate feedback.