Single Image Responsive in Post

Hi-
I have an image in a post that I want to make responsive. After checking out different options, I see lots of plugins for creating slideshows or galleries of multiple images that are responsive, but I just want to make one image responsive (because I only have one image in the post). I’m not an expert coder, and I’m wondering if there is a plugin to do this, or if there is some fairly simple code to use. Thanks!
BTW: I’m using the Ilisa theme
https://www.pixxels.at/wordpress-theme-ilisa/