The default is actually 75. The quality difference you see is more likely due to the image resizing algorithm that the PHP/GD library has (which is only bilinear, I think).
If you look in admin-functions.php, and look for function wp_create_thumbnail, this is where it’s making the thumbnail file.
Scroll down and you’ll see this code:
// move the thumbnail to its final destination
if ( $type[2] == 1 ) {
if (!imagegif( $thumbnail, $thumbpath ) ) {
$error = __( "Thumbnail path invalid" );
}
}
elseif ( $type[2] == 2 ) {
if (!imagejpeg( $thumbnail, $thumbpath ) ) {
$error = __( "Thumbnail path invalid" );
}
}
elseif ( $type[2] == 3 ) {
if (!imagepng( $thumbnail, $thumbpath ) ) {
$error = __( "Thumbnail path invalid" );
}
}
This is where it saves the thumbnail file. Note this line:
if (!imagejpeg( $thumbnail, $thumbpath ) ) {
That saves the image to a jpg file if needed. No quality is specified, so it’s using the default of 75.
To change it, edit that line like so:
if (!imagejpeg( $thumbnail, $thumbpath, 80 ) ) {
Where 80 is whatever compression you want (0-100).
