I have this expandable image viewer that I cant seem to change the size of the thumbnails in the viewer. What I want to achieve is the 2 thumbnails would fill the width of the screen the first being 40% the second being 60% and when clicked it expands and reveals the other half of the first thumbnails image but actually setting a width to the thumbnails is where I'm having trouble with.
A mockup of what I want to make:

Jsfiddle of my progress:
https://jsfiddle.net/Dunne08/8pd7brtx/24/
Any help would be greatly appreciated