Can I optimize my images if they are stored on an external server?
ShortPixel Image Optimizer (SPIO) can optimize images only if they are stored on the server where the plugin is installed and they are publicly accessible. This is because of the way the plugin works.
The only exception to this is the plugin WP Offload Media or S3 Uploads. SPIO officially supports them, therefore the only way you can optimize images on an external server is if they are offloaded using one of the previously mentioned plugins.
Important: Even if you use WP Offload Media or S3 Uploads, the files need to have a public ACL (Access Control List), otherwise ShortPixel won't be able to optimize the images. For more information, please consult this article.