Custom image size is not recognized

The plugin does not recognize the custom image size added by add_image_size method, which is typically called in the functions.php of themes.
Since the plugin is loaded before the theme and it handle the upload request on load, the custom image sizes added by the theme are not recognized nor generated.

It would be great if the plugin handle the upload request more later using action hook to support custom image size.

Thanks

https://www.ads-software.com/plugins/pixabay-images/