Thank you both for your answers. Ideally, I'd like to retain the embedded text in picture and just keep the 2 testimonial pictures small. I'm going to try out ArminB's answer to see if I can get it working.
Edit - Hmmm, I can't seem to get ArminB's solution working. Any suggestions? Gonna keep trying.
Edit 2 - I found a very simple way to do this as long as you have a large image to work with. It turns out that I can drastically reduce the image size using edit - edit image, without affecting how it displays on the web. Squarespace won't automatically blow up images so I just had to make them smaller than the resolution of the mobile device.Once I got below ~150px they did start to get smaller on a FHD screen but they still look fine, and, most importantly, they now look great on mobile devices.