Creating a modal previous and next interaction for images [on hold]

I am having trouble adding a functioning previous and next button to my modal window to allow users to navigate through my images without clicking the X and finding the next image. Here is my website link, so you can see the code, and how everything is set out. http://monospaced.co.uk/

I have been struggling for awhile, my last test I managed to get the previous and next buttons, but they were sitting on the main page when i need them inside of the modal window for each image, and it wouldn't switch. Thanks, for taking the time to help. (if anyone does!)