Blog images shrink too much on small screens

Helping a friend set up a WordPress site. We’re both fairly new to it.

One strange problem we’ve run into is that images in blog posts shrink when viewed on a small smartphone screen.

It starts out at full size, and then shrinks down a little. Sometimes it stops there, sometimes it keeps shrinking and shrinking until it completely disappears. We’ve tested on several different smartphones. The results are irregular, but they all seem to do it sometimes.

This is the page that’s doing it:
http://melanieregier.com/website-woes/

We’re using the Sketch theme as a base.