<?php
if ( function_exists( 'add_theme_support' ) ) { 
    add_theme_support( 'post-thumbnails' );
    // set_post_thumbnail_size( 150, 150, true ); // default Post Thumbnail dimensions (cropped)

    // // additional image sizes
    // // delete the next line if you do not need additional image sizes
    // add_image_size( 'category-thumb', 300, 9999 ); //300 pixels wide (and unlimited height)
}
?>