Enlarge image to maximum possible size on click

Hello. In a page of my website I included an image which I’d like to be enlarged to full screen (maximum size possible) when clicked.

I used Lightbox to achieve that but when I click on the image and the Lightbox popup activates, the images doesn’t go full screen, it stays more or less the same size as it is on the page.

The image is included in the page with a size which is smaller than its actual size. The chosen size for the thumbnail on the page is 1150×221 px, while the actual size of the picture is 1920 x 300, more than anough to go full screen even on a big screen (mine for example has a resolution of 1680×1050).

Can anybody help me? Thanks in advance.