I have similar issue in customiser. In my case, I’m using Masonry script, which seems to be affecting it. Interestingly, if I insert a sample item using an external image link just like below:
<?php if ( is_customize_preview() ) : ?>
<img src="https://wikipedia.org/any-sample-image/" />
<?php endif; ?>
Then this image shows without a problem, while all other post-retrieved images do not. As @peter Cralen said, image is always there, the height is set to zero. So far, if I disable Masonry script, the image shows up again. It happens only in Safari, all other browser seem to be unaffected.
