Our in house design team suggest these tips for images inside lessons:
It's not the sizing that actually helps. You want to set the width to 100% (not pixels) and leave the height blank. That makes it full size on mobile but doesn't change the shape. It makes it really big when viewing the lesson on the web but there isn't a great way to get both to look perfect.
We set the images in programs to a width 400px. I always delete the height.
If you are getting skewed images, it is possible setting both dimensions separately (without locking them) and is distorting the ratio.