Important: All scripts hosted on widcraft.googlecode.com don't work anymore because Google has blocked that SVN repository.
According to some blogs, WordPress automatically compress your JPEG images to 90% quality, but this information is false.This only happens on the resized images WordPress creates. The original uploaded image is saved unaltered, as the "full" size.
Anyways, you can stop compressing of JPEG images by putting following code into your functions.php file:
add_filter('jpeg_quality', function($arg){return 100;});
add_filter( 'wp_editor_set_quality', function($arg){return 100;} );
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