There could be a new icon in the Image Tools. This icon would resize a selected image (or several ones) to 25% - 33% - 50% - 66% - 75%
A checkbox next to this button would enable or disable the creation of a new file for this task.
If checkbox disabled, the resize tast would affect the current selected file.
If checkbox enabled, a new file would be created. This would prevent to alter the original image file.
Mockup image : http://img15.hostingpics.net/pics/406325ScreenShot002.png