Changes to image are permanent without hitting save
Changes to files are saved automatically when you close, or automatically in the background.
If you don't want to alter the original image, duplicate it first with ⇧⌘S (File → Duplicate), or use ⌥⇧⌘S Save as… (File → ⌥Save as…).

To revert to the original version, open the file and choose File → Revert To and select a revision.