You probably know this function needs to have GD enabled. If you dont know how just google how to enable it on your server.
Here is the function: imagecopyresampled(destination_resource,image_resource,destination_x,destination_y,src_x, src_y, destination_width,destination_height,src_width,src_height);
Run down below of the parameters:
destination_resource = destination resource is the container of the image, meaning this will be the final dimension of your image and contains the actual image, think of it as a canvas for your paint. When you output the image its the box that will render it. This usually comes from imagecreatetruecolor,imagecreate etc and other functions. specifing the width and height.
image_resource = is of course the resource image itself. Kinda like the paint you will use on the canvas this is the IMAGE you want to put on the container or canvas or the destination resource. This comes from imagecreatefromjpeg,imagecreatefrompng specifying the path to the image.
Now the rest of the parameters we usually do not understand.