Featured Image Thumbnail Creation

Question

What function or script creates the hard-cropped feature image when a post is saved? Regardless if I stipulate a size when saving the featured image in the media library (like the medium size), a new image is created upon saving the post.

Answer 1

The image isn't created on save, but rather on upload. The same process that creates the "thumbnail", "medium", and "large" size images also creates any other image sizes defined via add_image_size().