Yes! Just open the Image Optimizer app, click 'Settings' in the app and click 'Select This Plan' under the plan you would like. Follow the Shopify prompts from that point.
You also can learn more about Shopify recurring app charges here https://help.shopify.com/en/manual/your-account/manage-billing/your-invoice/apps#recurring-app-charges.
If you have any questions, feel free to send an email. We are here to help you and happy to answer any questions! Email: firstname.lastname@example.org