Left hand image?

Maybe a Youtube video would help? https://www.youtube.com/watch?v=CZ7Rs1bz95M

There’s also the documentation here: https://sewpafly.github.io/post-thumbnail-editor/editor/

Does that help?

Thanks for the answer. But I’m still unable to use the plugin.
It clarifies that the plugin does not work as it should on my wp.
It’s 4.0, maybe that’s it?

The left hand image (40 seconds in on the youtube video) never appears, only the right hand ones, after choosing thumbnail size. Which I can’t crop, of course. Should the images be a special format? Most of mine are png?

It should work on 4.0. (I have scattered reports of it not working in Safari), but the problem you’re reporting is common enough, and it should have a pretty simple fix (but could take some time). See these threads:

• https://www.ads-software.com/support/topic/editor-breaks-with-buddypress?replies=11#post-4738813
• https://www.ads-software.com/support/topic/plugin-post-thumbnail-editor-no-image-on-the-left-to-crop?replies=38#post-2698807
• https://www.ads-software.com/support/topic/admin-ajaxphp-imgedit-preview-header-problem?replies=4#post-3252437

TL;DR; Probably a conflict with another plugin. Try enabling/disabling plugins to see which plugin is giving you grief (it might be a theme too). Typically these problems occur when there is a PHP file with space after a closing ?> or before an opening <?.

Ok, thanks.
I’ll look into it when I find the time.