Hi,
I want to make sure you have some options available while we sort this out. If a current issue is causing you issues, you can always remove the plugin and download the version that you found that worked from here: https://github.com/presscustomizr/nimble-builder/releases
However, what you might look to do, is to make sure that everything, including this plugin is fully updated. Usually when photos disappear it is the result of a conflict with a plugin. Once everything is updated, you can deactivate all plugins and see if photos work. Then, add one back in at a time until you notice the conflict. From there, we might be able to assist.
Also, when your photos disappear, can you tell me specifically what you mean? Are they gone from your website, or just the editing UI? Do they just disappear from the editing interface or are they gone from the media library also?
The more information I have, both about your problem and your current themes/plugins, the more I can try to recreate the problem to attempt to solve it.
Thanks!