Any hidden images will show up as an option any time a multi-select Pinterest tool is used, such as the Pinterest browser extension or a social sharing plugin that uses the multi-select tool. However, if that doesn't work, the culprit might be one of the following:
Page builders/Lazy load plugins
If you're using a page builder or lazy load plugin, note that those can sometimes conflict with hidden images.
Page builders are great for building out complex page layouts when you need design elements that the standard Block Editor doesn't provide. However, for general content creation, WordPress' Block Editor is going to be the most reliable and well-supported editor out there, so that is what we recommend using for content creation.
As for lazy load plugins, we recommend entering the appropriate snippet from the following support article to resolve the issue: https://support.wptasty.com/en/articles/1484582-how-can-i-make-the-hidden-image-work-with-lazy-load
Your thumbnail size
The issue might be related to the pinning tool not picking up thumbnails that are smaller than 151x151px (the default WordPress thumbnail is 150x150px).
To troubleshoot this, navigate to your thumbnail settings (Settings > Media) and change the dimensions so they're at least 151x151px.
When thumbnail settings are changed, thumbnails aren't regenerated for previously uploaded images, so you'll need to use a plugin like Regenerate Thumbnails to make all the new thumbnails.
If you do not want to change your thumbnail size, you can specify that a larger image be used by adding a filter to your functions.php file Here's more on that: https://support.wptasty.com/en/articles/2281001-how-do-i-change-the-size-of-the-hidden-image-thumbnails