Users can now delete original fullsize image while leave the auto-resized image to save space. Here's how:

1. login your account
2. visit one of the images of your target album
3. click "delete original"
4. select if you want to delete ALL fullsize image or just the image you selected.
5. click "submit" to confirm
6. Saved space will reflect at your used quota value.
7. If there's no full version, nothing will do at your images, so, no harm.