Just indicate where you want to save the file
imagejpeg($crop,$FILENAME);
EDIT
Based on your comment below, the problem is not saving the file, but that you do not have the correct image resource.
$image = imagecreatefromjpeg($filename);
it should be
$image = imagecreatefromjpeg($filename); if (!$image) exit("not a valid image");
You tell him to copy $ 250,300 $ x, $ y from an orignal image to a new image, but there was no check to make sure you have an image that is large enough to do this, or indeed that x and y exist or that they are less than 250 300
Also you are currently trying to use imagedestroy() file name. You can only imagedestroy image resource.
imagedestroy($image); imagedestroy($crop);
source share