Ah, you are right the image gets all stretched out! Everything you said worked, thanks very much for your help!
Does this mean that I can only upload images with a width of 1200px or is there something I can adjust somewhere int he CSS file? I’m a complete novice to this stuff, but ideally I wanted post images to appear as they do under the ‘Radcliffe’ theme (https://wordpress.org/themes/radcliffe), where no adjustment is necessary to images to make them take up the whole width of the page. The only downside to that, however, is that only images show up in the home page, and no text. A wide image and a little bit of text in the Hoffman theme, as it appears in the demo, would be ideal. Would you have any ideas on how to fix? Thanks so much!