You are just creating your array incorrectly. You could use http_build_query:
$fields = array(
'username' => "annonymous",
'api_key' => urlencode("1234"),
'images' => array(
urlencode(base64_encode('image1')),
urlencode(base64_encode('image2'))
)
);
$fields_string = http_build_query($fields);
So, the entire code that you could use would be:
<?php
//extract data from the post
extract($_POST);
//set POST variables
$url="http://api.example.com/api";
$fields = array(
'username' => "annonymous",
'api_key' => urlencode("1234"),
'images' => array(
urlencode(base64_encode('image1')),
urlencode(base64_encode('image2'))
)
);
//url-ify the data for the POST
$fields_string = http_build_query($fields);
//open connection
$ch = curl_init();
//set the url, number of POST vars, POST data
curl_setopt($ch,CURLOPT_URL, $url);
curl_setopt($ch,CURLOPT_POST, 1);
curl_setopt($ch,CURLOPT_POSTFIELDS, $fields_string);
//execute post
$result = curl_exec($ch);
echo $result;
//close connection
curl_close($ch);
?>