Returns an image resource identifier on success, FALSE on errors.
I had it return False on an image that was manually re-named to a JPG, when it was actually a PNG. I got an error saying:
'not a valid JPEG file'
You should look to see what error you get when performing that function, That way you'll know exactly what's goind bad.
One way to check for the error is creating a PHP file next to the image with:
<?php
echo imagecreatefromjpeg("path/to/image");
?>
run it in using 'php filename.php'
and looking in the console for the result