What I'm trying to do is load a website on a frame and then mark points on it depending on screen coordinates obtained via a function.
Do I need PHP GD?
Loading the website on the frame is not an issue. It's how to mark points on the screen that's causing my hair to fall
Nah, what you need are absolute positioned HTML elements.
You can grab the page with PHP with curl or file_get_contents() and inject some HTML/CSS into it that creates absolutely positioned <p>s or <span>s or whatever.
Checkout this post here. It uses Mootools(a JavaScript framework), PHP, and MySQL. I think it's very similar to what you are trying to do except you want to mark points on the iframe instead of doing it on an image. The plugin can be attached to an element: