Can't edit stock levels

[Guest autumnmoon]

cube cart was working fine until today. When i went to edit stock levels i got this error:

Fatal error: Call to undefined function: imagecreatefromjpeg() in /home/autumnmo/public_html/store/classes/gd.inc.php on line 48img

here is the string i think it refers to

img["format"]=ereg_replace(".*\.(.*)$","\\1",$imgfile);

$this->img["format"]=strtoupper($this->img["format"]);

if($config['gdversion']>0)

{

if ($this->img["format"]=="JPG" || $this->img["format"]=="JPEG")

{

//JPEG

$this->img["format"]="JPEG";

$this->img["src"] = imagecreatefromjpeg($imgfile);

ive not changed anything so am at a loss as whats happened, anyone know what i need to do to get it working again.

I have removed the product image which will then enable me to edit the stock level, but when I add the image back to the product I get the same error!

This is doing my head in and also costing me business as I can't show new items in stock!

Please note I am not technically minded at all - so in giving a response could you please make it a 'for dummies' answer with step by step instructions. Thanks a lot.