I know what you mean. It is a little hacky. The issue is WPThumb... I would like to crop the image to size if it is active, yet still work when it is not (using defined image sizes instead - default is 'thumbnail') However the arguements required are slightly different depending on whether it is active or not.

    // Works with WPThumb, but breaks if it is not active.
    echo wp_get_attachment_image( $id, array( 'width' => 100, 'height' => 100, 'crop' => true ) );

    // Format expected by WordPress (without WPThumb). 
    echo wp_get_attachment_image( $id, array( 100, 100 ) );

    // This works when WPThumb is active or not. the crop arg is just ignored.
    echo wp_get_attachment_image( $id, array( 100, 100, 'crop' => true ) );