you are resizing from 1024x768 to 150x113 ?
the first thing you'd want to do is to create one image at 150x113 using the
$thum = imagecreatetruecolor(150,113);
syntax
then you want to create a copy of your resource image with maybe
$reso = imagecreatefromjpeg('image.jpg');
then you copy resource (now called $reso) onto thumbnail (called $thum) with a size change
imagecopyresampled( $thum, $reso, 0, 0, 0, 0, 150, 113, 1024, 768);
now $thum looks like a small copy of $reso - so you output it
imagejpeg($thum, 'thumbs/image.jpg', 85);
85% compression is pretty good for thumbs imo.
If you are feeling daring and want to take your thumbnailing a step further - try downloading the class I wrote at -
http://www.phpclasses.org/browse.html/package/1007.html
and follow the comment suggestions. Does all the hard work for you and can also do drop_shadow, bevel, ellipse, merge, greyscale, motion_blur and more.
Subnotes:
GD2+ allows use of truecolour jpeg and png images, so you'd always want to use imagecreatetruecolor and imagecopyresampled in place of imagecreate and imagecopyresized (the latter two only support palette images - ie 256 colour)
the class I linked is 'specially designed for GD2+
