Stop images from scaling to window size in lightbox.

I'm using grids to disply galleries on my site and when the image is clicked a lightbox opens with the image and details. Great. BUT when the image opens in the lightbox is scales up to the size fo the window.

How can I stop the images in lightbox being resized? I only want them to display at the size I uploaded them in.

Code n00b here, so the less complex the solution the better.