Yes, on the fly. Leave the plug in activated. But even if you deactivate it, a web page with a picture still works fine. It simply displays next closest default size or the original size. It's not a big issue as the original size is properly downsized.
I wonder why it’s not discovered and appreciated by website owners. This plugin is a must-have.
1. It benefits anyone who use photos on their websites. Imagine how much storage you can save by downsizing a 12M photo to less than 500K while keeping the clarity of full high definition.
2. While there are WP plugins for users to choose or set face area manually for cropping, this is the only one that put face detection algorithms which have been there for 4 years in use. Imagine how the time or hassle you can save when uploading each photo? Yet it has an API to take the data from the manual plugins and better algorithms in the future.
3. Many site owners are annoyed by those thumbnails automatically generated no matter needed or not. This plugin cleans up them all. Imagine how lean and clean your media library will be.