An action of adding image to product variant is missing. You can make custom webhook calls to add image to product variant. Below is the API doc of Shopify to add image to product variant. 

https://shopify.dev/api/admin/rest/reference/products/product-image#create-2021-07

Hope this helps.

I just wanted to jump in here as spotted that there’s an existing feature request for the ability to set an image when using the Create Product Variant Shopify action. Which was also being discussed here:

I’ve added @KieraC’s vote for that feature request. We don’t have an ETA but will be sure to send an email notification and update that other thread in Community as soon as it’s added. So please subscribe to that other topic to be kept updated on any news or workarounds we may find!

In the meantime, it might be possible to use a Webhook step, as jayeshkumarbhatia mentioned, to create the product variant and image. :)