Problem with ImageCreateFromJPEG

I’m currently doing a image reconition class, but I get getting the error :

Fatal error: Call to undefined function ImageCreateFromJPEG() in J:WEBSITEWWWclassRandomImageText.php on line 27

The code on line 27 is the Line with the ImageCreateFromJPEG :

[code=php] function RandomImageText($jpeg, $fheight = 10, $fWidth = 10)
{
$this->image = ImageCreateFromJPEG($jpeg);
$this->iHeight = ImageSY($this->image);
$this->iWidth = ImageSX($this->image);
$this->fHeight = $fHeight;
$this->fWidth = $fWidth;
$this->xPos = 0;
$this->fonts = array(2, 3, 4, 5);
}[/code]

Can anyone give a hand ?

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