Many premium WordPress themes come with custom image sizes for your posts and custom taxonomies. You can always disable or change them with a bit of coding. Simple Image Sizes is a plugin that lets you create and manage custom image sizes for your site more conveniently.
The plugin not only allows for custom image sizes, it also overrides your theme’s options. Users have the option to target only a limited number of images. Simple Image Sizes also lets you know when you can expect the regeneration process to end. This is a free script.