将动画GIF转换为PNG

我试图只将动画GIF的第一帧显示为PNG，用作缩略图。我现在有这个代码：

<?php
function myImageCreateFromGif($file_or_url) { 
$dummy_file = "/tmp/dummy.gif"; 
# if this is a url, use fopen to get the file data, then 
# save it to a dummy file 
if (preg_match("/(http|ftp):'/'//i", $file_or_url)) { 
        # open the file using fopen, which supports remote URLs 
        $input = fopen($file_or_url, "rb"); 
        # read the contents of the file 
        # will accept files up to 10Mb, but will probably get 
        # and EOF before that, we have to do it this way because 
        # filesize isn't designed to work with URLs.  sigh. 
        $image_data = fread($input, 10000000); 
        fclose($input); 
        # write the contents to a dummy file 
        $output = fopen("$dummy_file", "wb"); 
        fwrite($output, $image_data); 
        fclose($output); 
        # create the gif from the dummy file 
        $image = ImageCreateFromGif($dummy_file); 
        # get rid of the dummy file 
        unlink($dummy_file); 
    } 
    # if it's not a URL, we can simply open the image directly 
    else { 
        $image = ImageCreateFromGif($file_or_url); 
    } 
    if ($image) { return $image; } 
    else { return 0; } 
}
$image = "http://i.imgur.com/".$_GET["i"].".gif";
$img = myImageCreateFromGif($image);
if($img) {
    header("Content-Type: image/png");
    ImagePNG($img);
    ImageDestroy($img);
}
?>

这很好，但GIF在变成PNG之前不会完全加载，所以如果GIF根本无法加载，页面会返回一个损坏的图像，或者是一个部分加载的GIF，通过重复加载来填充未加载的部分。那么，在将GIF制作成PNG之前，我如何才能将其完全加载呢？