By default in WordPress when you server up a .JPEG file, WordPress optimizes the image to save bandwidth and space. 90% of the time, no one will realize this as the WordPress function behind image optimization does a fantastic job at keeping it’s quality the same, unless you are using very large high quality photo .jpeg files.
By default WordPress uses a default quality of 90%, if you wish to revert the image quality throw the following function in your functions.php file:
add_filter( 'jpeg_quality', 'smashing_jpeg_quality' );
function smashing_jpeg_quality() {
return 100;
}