Fitting content to not overflow /be viewable on sidescrolling site

Hello - the code I am working on is for my Portfolio Project. I wanted to make it a side scrolling site and found helpful instruction on how to achieve that here.
The biggest problem I am having is that My images are way larger than the screen, and that’s rather pointless on a website that is designed to show images! How do I fit the container/images/content to fit into each particular “slide” on the site? I tried giving a class of “slide” to each container:

.slide {
width: 100vw;
height: 100vh;

but its still overflowing and unviewable. It looks about right when I preview the entire site at 0.25%. I just don’t know where to go from here!