function custom_theme_setup() {
    // Define custom image sizes for responsive design
    add_image_size( 'responsive-small', 320, 0, true );   // Small for mobile
    add_image_size( 'responsive-medium', 640, 0, true );  // Medium for tablet
    add_image_size( 'responsive-large', 1280, 0, true );  // Large for desktop
    add_image_size( 'responsive-xlarge', 1920, 0, true ); // Extra large for high-res screens
    add_image_size( 'responsive-xxlarge', 2560, 0, true ); // XXL for high-res monitors and 4K
    add_image_size( 'responsive-4k', 3840, 0, true );     // For 4K displays
}
add_action( 'after_setup_theme', 'custom_theme_setup' );
