Hey @romadov,
Thank you for reaching out to us for support. Make sure you have the latest version of the plugin, 2.5.4, you should then not have any issues displaying any post thumbnails. If you are, please ensure you have Favor Local Images enabled under Instagram Feed > Customize, and you may also need to click Reset Resized Images as well as Clear Backup/Permanent Cache to force the plugin to get images with the new system. It may also be a caching issue from other plugins so make sure to clear any website and local cache. Let us know should you still be having issues and if possible send a link to the website in question.
Best regards.
