I've found some similar questions to mine, but none that address my specific situation, so I thought I'd ask.
What I have: A square JPEG, measuring 600px x 600px, that I am getting from an external source.
What I want: To programmatically resize the square to 400x400, and add 75px of white space on each side, making the total final image 550px x 400, with the original square image centered.
I'd be grateful if somebody can point me in the right direction.
php.net/manual/en/ref.image.php
These functions are be able to do exactly what you want, and they are pretty simple to use as well.