for this page, I'm having trouble with how the page displays on different sized monitors, even just on desktop. How can I get the entire main image to display above the fold dynamically? Thanks!
You can use 'vh' units. Read more about 'vh' units. see supported browsers
Example (your page):
I added height: 100vh;
to <figure id="thumbnail" >...</figure>
and height: 100%; width: 100%;
to the <img>
inside it (removing all other style you had there) - it did the trick