WordPress does do some compression on images when it resizes them. You can disable this very simply using the following code (or using a plugin like this one):
function wpse_114909_image_quality() {
return 100;
}
add_filter( 'jpeg_quality', 'wpse_114909_image_quality' );
I’m sure that GD & Imagemagick are not as good at resampling images as Photoshop is, so this may be a battle you’ll never win. All the same, this will certainly help. For any images where the quality matters that much to you, you should manage the images manually instead of letting WordPress resize them.