IH uses the base image (whatever you load into the images directory) to create the small and medium images... but the large image is rendered at whatever size and resolution that your uploaded image is...

In other words, if you load up a 5mb image, or 2400px X 2400px - than while your SMALL and MEDIUM images will be re-created by IH, your LARGE image will still load as a 5mb file, and will display in the popup as 2400 X 2400.

So... it is still necessary to optimize your images.

I try to keep all large images under 100kb, and 600px X 450px .