[TigerC10]

GD is server side image resizing. This means a lot more work for your server to do when it loads the page, but it also means that more people can view the page.

The alternative is JavaScript image resizing. This is all client side, which means that it's less intensive on your server (this good especially if you're on shared hosting). However, because it relies on javascript it means that not everyone can get the resize (really ridiculously old browsers may not support javascript, likewise with mobile phone browsers).

So ultimately you have to ask yourself, "more intensive on my server? or less intensive but not guaranteed to work 100% of the time?"

Personally, I prefer Javascript. The majority of browsers that access your page will be compatible with javascript. But some people don't like to chance it, that's why they use GD (and if I had a server farm dedicated to 1 website, I'd probably use GD).