When you are trying to upload a file . file get always uploaded using post. So you need to place @ before the file name which you want to send as a post. Using @before the file path in order to make sure that CURL sends the file in the form of a multipart/form-data post . This can be because api may not return anything while successful uploading.

 $ch = curl_init("http://imagezilla.net/api.php?apikey=''&testmode=1");
 $post = array(
    "file"=>"@C:\penguins.jpg",
);
 curl_setopt($ch, CURLOPT_HTTPHEADER, $header);
 curl_setopt($ch, CURLOPT_POST, 1);
 curl_setopt($ch, CURLOPT_POSTFIELDS, $post); 
 curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
 $output = curl_exec($ch);       
 curl_close($ch);
 echo $output;