I think the problem is that the image size isn’t know when the popup opens. It’s only determined after the image downloads.
Although the images were inserted with a defined height, the theme’s CSS is setting it to ‘auto’:
img { height: auto }
You could either remove the line above, or in your style.css, you could override it by adding something like this:
.mapp-body img { height: 100px !important; }