how do i prevent summary block images from getting too big?

Hi- Looking for some simple CSS help :)I've looked through some questions already and didn't find my answer so apologies if I am being redundant. I have a simple slideshow with one image displayed at a time. The images were created 1000 px square and I don't want them to get larger than that. I thought of putting spacers on the side but I don't want them too small on smaller screens. I am hoping there's some way to just say "don't get bigger than 1000 px but smaller is okay." in code.https://www.nokillmag.com/home22/3-things-to-buy-to-start-off-plastic-free-july is the page. thank you