New event image generation broken #2563

Description

ref #2496

I followed the steps in https://handbooks.graphql.org/locals/social-media#event-banner-images

I have ended up with three pngs created in social-banners but they seem corrupted or broken. I am unable to open these in an image editor. Opening in a web browser gives the message "This image at [filepath] cannot be displayed, because it contains errors."

Is there a missing step or missing assets?
Is it because I am using a Mac?