README

This is more of just a proof-of-concept, build off it, it currently has several potential security flaws and is completely untested.

At least it should give you a rough idea of one way to handle the skin upload stuff.

accept.php

<?php
if(!isset($_POST['username']) && !isset($_POST['image'])) {
    die("haaaaaxxx!");
}

$username = $_POST['username']; //sanity-check
$decoded = bade64_decode($_POST['image']);
$handle = fopen($username . ".png", 'wb'); //open user image in binary mode
fwrite($handle,$decoded);
fclose($handle);
echo("OK");
?>

skinupload.php

<?php
//have a html form with a file upload named "skin"
//obviously you should sanity-check that it is a PNG file and stuff, this is just concept.

//
// IMAGE ENCODING
//

$handle = fopen($_FILES['skin']['tmp_name'], "r"); //open uploaded temp file
$imgbinary = fread(fopen($imgfile, "r"), filesize($imgfile)); //read the file contents
$encoded = base64_encode($imgbinary); //encode with base64
unset($imgbinary); //
fclose($handle); //free memory
unset($handle); //

//
//FILE TRANSFER
//
$skinscript = "http://skins.evilminecraft.tnet/accept.php";
$data = array (
    "username" => $_SESSION['username'],
    "image" => $encoded
);

$ch = curl_init ($skinscript);
curl_setopt ($ch, CURLOPT_POST, true);
curl_setopt ($ch, CURLOPT_POSTFIELDS, $data);
curl_setopt ($ch, CURLOPT_RETURNTRANSFER, true);
$returndata = curl_exec($ch);

if($returndata!="OK") {
    //failed
} else {
    //win!
}
?>