Does this happen with all large images, or just large images that you're viewing through your web browser?
Most browsers will automatically resize large images so the entire image fits on the screen without you having to scroll around to see it, but the algorithm they use makes the resized image look like crap.
If you're using IE 7 or Firefox, the cursor should turn into a magnifying glass with a plus sign when you hover over the image with your mouse, and clicking it will display the image in it's native size. If you're using IE 6, an "expand to regular size" button should appear in the bottom right of the picture when you hover over it.
If this is the problem, let me know what browser you're using and I should be able to tell you how to turn off the automatic resize.