rainborick - 3:01 pm on Jun 1, 2012 (gmt 0)
Generally, the advice I see suggests using imagecopyresampled() for better quality. Choosing an appropriate output file type can also often help, especially if you're using a file type that supports a quality setting like .jpg.
I don't have any real expertise with image manipulation, but I've gotten better results using ImageMagick for these tasks than I did with GD. There are a couple of PHP interfaces like IMagick that make it easy, but my current set-up makes that impractical so I just use system() and passthru() to execute the commands I need. For the simple tasks I have, it works fine. Good luck!