$url = 'http://twitter.com/account/update_profile_background_image.xml';
$uname = 'myuname';
$pword = 'mypword';
$img_path = '/path/to/myimage.jpg';
$userpwd = $uname . ':' . $pword;
$img_post = array('image' => '@' . $img_path . ';type=image/jpeg',
'tile' => 'true');

$opts = array(CURLOPT_URL => $url,
CURLOPT_FOLLOWLOCATION => true,
CURLOPT_RETURNTRANSFER => true,
CURLOPT_HEADER => true,
CURLOPT_POST => true,
CURLOPT_POSTFIELDS => $img_post,
CURLOPT_HTTPAUTH => CURLAUTH_ANY,
CURLOPT_USERPWD => $userpwd,
CURLOPT_HTTPHEADER => array('Expect:'),
CURLINFO_HEADER_OUT => true);

$ch = curl_init();
curl_setopt_array($ch, $opts);
$response = curl_exec($ch);
$err = curl_error($ch);
$info = curl_getinfo($ch);
curl_close($ch);

echo '<pre>';
echo $err . '<br />';
echo '----------------' . '<br />';
print_r($info);
echo '----------------' . '<br />';
echo htmlspecialchars($response) . '<br />';
echo '</pre>';