getting blank 'temp_name' in image upload - php

Question

I'm working in my site which is in zen cart, actually a want to add a little functionality on it, and that is.... When I upload a image for a product I also want to create a thumbnail size of that image to be in a folder. Now I tried to search a file where the code to upload image goes on... I found a file in htdocs/admin_folder/includes/modules/new_product_preview.php
Now after getting details from user from a page htdocs/admin_folder/product.php it form post it in new_product_review.php page with a action defined in it and here the code for uploading image I've just simply added code as follows but it is not uploading the image in folder.
I've tried to echo the name of the file, so I've just put the alert for the image name parameters, I'm getting the $_FILES["products_image"]["tmp_name"] as something like, /tmp/gsnVaX.. like but
when I check $_FILES["products_image"]["name"] it gives the correct name of the file...
Why its not putting the image in temporary folder? When I put the resize code on it then it saves image in my folder with the same size which I wanted but whole image is just blank black.
Code is:
$image = "../images/thumbs/anki_".$_FILES["products_image"]["name"];
move_uploaded_file($_FILES["products_image"]["tmp_name"],$image);
and code when I'm doing this through resize is:
$control = "products_image";
$fileName = $_FILES[$control]['name'];
$uploadedfile = $_FILES[$control]['tmp_name'];
echo "<script>alert('".$fileName."');</script>";
$exts = explode(".",$fileName);
$ext = array_pop($exts);
$uploadedfile = $_FILES[$control]['name'];
$extension = $ext;
$extension = strtolower($extension);
if($extension=="jpg" || $extension=="jpeg" )
{
$src = imagecreatefromjpeg($uploadedfile);
}
else if($extension=="png")
{
$src = imagecreatefrompng($uploadedfile);
}
else
{
$src = imagecreatefromgif($uploadedfile);
}
$newwidth = 140;
list($width,$height)=getimagesize($uploadedfile);
$newheight=($height/$width)*$newwidth;
$newheight = 135;
$tmp=imagecreatetruecolor($newwidth,$newheight);
imagecopyresampled($tmp,$src,0,0,0,0,$newwidth,$newheight,$width,$height);
$filename = "../images/thumbs/". $fileName;
imagejpeg($tmp,$filename,100);
imagedestroy($src);
imagedestroy($tmp);
What am I doing wrong with it?