[Plugin NextGEN Gallery] Images being resized problem

I have the latest NextGEN gallery plugin and the latest WordPress running on a blog I am developing. I have altered the default size of the images from 800 x 600 to 600 x 450. I upload my images in this size. However, when I display a gallery the images are upsized to 800 x 600 and as a result they look terrible. Is there anywhere to force NextGEN not to perform this upsizing and to leave and display my images at the uploaded size?