Because it’s downsized (original 649px wide, displayed version 450px wide). Images downsized by WP lose some color and sharpness – see this post of mine: http://wpbtips.wordpress.com/2010/01/16/image-quality/
You should create and upload images that are in the pixel dimensions you want on the post (and insert them in full size). On the theme you’re using, the maximum you can use is 600px.
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