GD is still one of the more elusive things about PHP - people know that they can use it to manipulate images and create images for their websites in realtime, but they look at the functions and loose sight of what they were trying to do. Functions like imagecopymerge() or imagecreatefromjpeg() just seem to complex for them, so they give up, looking for another way.

Well, DevArticles.com is here once again to help you with your woes. Their newest article, Resizing JPEG's With PHP and GD, guides you through making a file upload that will resize your image to whatever you want the thumbnail size to be (form fields for inputting this).

They give you all the code and plenty of explanation about how it all works and why they did it that way. This can be a great thing to just "plug in" your site where ever you have a file upload - then, not only can you offer the thumbnails for people to view (easier time for the users to tell which image is which), but it can also make your site easier to work with and look nicer.