Disabling image thumbnails and automatic regeneration

Replacing the WordPress generated images using ftp is not at all a good idea and also would be tiresome for you.

Your problem is about the quality of the image

WordPress’ default compression ratio for image generation is not at 100%.

so you can make it 100% by putting the below code in the active theme’s functions.php file

add_filter( 'jpeg_quality', 'wpse_image_full_quality' );
add_filter( 'wp_editor_set_quality', 'wpse_image_full_quality' );

function wpse_image_full_quality( $quality ) {
    return 100;
}

This will only apply to thumbnails and images generated after you have inserted the code into your theme. If you want this to apply to existing images and thumbnails, you should use a plugin like Regenerate Thumbnails to create new, better quality images with the updated compression ratio.