I was trying to make some avatars out of some of my images and needed to resize them. I did so in Preview in Yosemite 10.10.3, and had before as well in Snow Leopard. There was no 'save as' option, so I did not save my changes because I didn't want to permanently alter my image. There was no 'undo' feature or 'cancel without saving changes' option either. What gives with this? I can't go back and take that picture again and now it is a low-res grainy mess when I try to resize it to original manually.
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.