I haven't used NetPBM but I would always rate IM over GD.
Why?
Memory useage and simplicity. GD, as part of PHP, uses PHP's allocated memory (usually 8mb) and if you've ever tried resizing an image with resolution much more than 1024x768 it is very easy to use it all up causing your whole script/page request to die.
IM on the other hand runs from the command line (via a shell_exec() call in PHP) so if you do run out of memory resizing a large image it won't take your php script with it.
If you are building a content management system for your self to use (or someone who knows how to use photoshop) GD is fine as you can shrink your image down to something acceptable and then upload it. However if your system is to be used by the computer illiterate who want to just upload images straight from their 4+ megepixel cammeras then you are going to have problems. Try telling them they can upload images upto 2mb but as long as they are 72dpi and no more than 1024x768pixels and you will quickly find your users can't be bothered and don't use your service, or you suddenly have your pop box jammed up with 10mb attachments from clients wanting you to do it for them. I have used my whole 8mb resizing a 200kb image before because it was 1600x1200px by the way.
IM, once you get the hang of it, is far simpler to use for image resizing, with GD you have to calculate the precise dimensions you want to resize your image to, requiring a few lines of getimagesize() and if(height>width) do this else do that etc etc. Then, once you have your dimensions, to actually resize the image is another 3 or four lines of creating image, filling it with your source image, resizing it, writing it to disk and then killing your temp image etc...
IM is just one line with a max height and width set and IM calculates the rest for you.
It may be that there is a better way to use GD that I don't know about, so it doesn't use huge amounts of memory, I did a couple of days of web research to get my GD script to resize large images and gave up. It then took about 3hrs to research IM and get it working perfectly.
Hope this helps someone,
RHBA
