@SetLikeJelly, it looks good. Some things you can do;
you’re failing one of the tests. Go back and clear that up
run your HTML through a validator. codepen provides you with one. Click on the arrow in the upper right and then click on the 'Analyze link. (There’s one for CSS and JS too) Clean up your HTML.
I’m using Chrome on Windows (same on Firefox though) and the image is not centering so it is showing as failing a test.
Instead of trying to center it by using the margin set to auto, ever think of setting the img-div display to use flex and use that to align your image - will make life totally easier for yourself!
@SetLikeJelly
You can make everything stack one on top of the other in mobile view because the images shrink too much. View the page on your cell phone.
Good job on your first webpage!
I did not do as well as you did on my first web page that I made a few years ago. I used a table for the layout which was a really bad idea.