Images uploader sizes…

How do I force the image uploader to make the medium and large images if the full is the same size. At the moment if I upload a 600 x 600 full image no medium or large is created as they are both set to 600 x 600. This makes sense not to make or do the extra work since the image is already the right size, but I want the additional copies so I can manually crop them using the new image editor and use the crops else where on the site. If I crop the full image I lose the original.