function get_avatar_url($get_avatar){
    preg_match("/src='(.*?)'/i", $get_avatar, $matches);
    return $matches[1];
}

<img src="<? echo get_avatar_url(get_avatar( $curauth->ID, 150 )); ?>" align="left" class="authorimage" />

$avatar = "<img alt='{$safe_alt}' src='{$default}' class='avatar avatar-{$size} photo avatar-default' height='{$size}' width='{$size}' />";

apply_filters('get_avatar', $avatar, $id_or_email, $size, $default, $alt);

if ( ! function_exists( 'get_avatar' ) ) :

/**
 * Retrieve the avatar for a user who provided a user ID or email address.
 *
 * @since 2.5
 * @param int|string|object $id_or_email A user ID,  email address, or comment object
 * @param int $size Size of the avatar image
 * @param string $default URL to a default image to use if no avatar is available
 * @param string $alt Alternate text to use in image tag. Defaults to blank
 * @param boolean $url, true for get only the url of the image, no markup
 * @return string <img> tag for the user's avatar
*/
function get_avatar( $id_or_email, $size = '96', $default = '', $alt = false, $url = FALSE ) {
    if ( ! get_option('show_avatars') )
        return false;

    if ( false === $alt)
        $safe_alt = '';
    else
        $safe_alt = esc_attr( $alt );

    if ( !is_numeric($size) )
        $size = '96';

    $email = '';
    if ( is_numeric($id_or_email) ) {
        $id = (int) $id_or_email;
        $user = get_userdata($id);
        if ( $user )
            $email = $user->user_email;
    } elseif ( is_object($id_or_email) ) {
        // No avatar for pingbacks or trackbacks
        $allowed_comment_types = apply_filters( 'get_avatar_comment_types', array( 'comment' ) );
        if ( ! empty( $id_or_email->comment_type ) && ! in_array( $id_or_email->comment_type, (array) $allowed_comment_types ) )
            return false;

        if ( !empty($id_or_email->user_id) ) {
            $id = (int) $id_or_email->user_id;
            $user = get_userdata($id);
            if ( $user)
                $email = $user->user_email;
        } elseif ( !empty($id_or_email->comment_author_email) ) {
            $email = $id_or_email->comment_author_email;
        }
    } else {
        $email = $id_or_email;
    }

    if ( empty($default) ) {
        $avatar_default = get_option('avatar_default');
        if ( empty($avatar_default) )
            $default = 'mystery';
        else
            $default = $avatar_default;
    }

    if ( !empty($email) )
        $email_hash = md5( strtolower( trim( $email ) ) );

    if ( is_ssl() ) {
        $host = 'https://secure.gravatar.com';
    } else {
        if ( !empty($email) )
            $host = sprintf( "http://%d.gravatar.com", ( hexdec( $email_hash[0] ) % 2 ) );
        else
            $host = 'http://0.gravatar.com';
    }

    if ( 'mystery' == $default )
        $default = "$host/avatar/ad516503a11cd5ca435acc9bb6523536?s={$size}"; // ad516503a11cd5ca435acc9bb6523536 == md5('unknown@gravatar.com')
    elseif ( 'blank' == $default )
        $default = includes_url('images/blank.gif');
    elseif ( !empty($email) && 'gravatar_default' == $default )
        $default = '';
    elseif ( 'gravatar_default' == $default )
        $default = "$host/avatar/?s={$size}";
    elseif ( empty($email) )
        $default = "$host/avatar/?d=$default&s={$size}";
    elseif ( strpos($default, 'http://') === 0 )
        $default = add_query_arg( 's', $size, $default );

    if ( !empty($email) ) {
        $out = "$host/avatar/";
        $out .= $email_hash;
        $out .= '?s='.$size;
        $out .= '&d=' . urlencode( $default );

        $rating = get_option('avatar_rating');
        if ( !empty( $rating ) )
            $out .= "&r={$rating}";

        if ( $url )
            $avatar = $out;
        else
            $avatar = "<img alt='{$safe_alt}' src='{$out}' class='avatar avatar-{$size} photo' height='{$size}' width='{$size}' />";
    } else {
        if ( $url )
            $avatar = $out;
        else
            $avatar = "<img alt='{$safe_alt}' src='{$default}' class='avatar avatar-{$size} photo avatar-default' height='{$size}' width='{$size}' />";
    }

    return apply_filters('get_avatar', $avatar, $id_or_email, $size, $default, $alt);
}

function get_gravatar_url( $email ) {

    $hash = md5( strtolower( trim ( $email ) ) );
    return 'http://gravatar.com/avatar/' . $hash;
}

// In your template ...
$avatar_url = get_avatar_url ( get_the_author_meta('ID'), $size = '50' );

// Get src URL from avatar <img> tag (add to functions.php)
function get_avatar_url($author_id, $size){
    $get_avatar = get_avatar( $author_id, $size );
    preg_match("/src='(.*?)'/i", $get_avatar, $matches);
    return ( $matches[1] );
}

preg_match("/src=['"](.*?)['"]/i", $get_avatar, $matches);